REESE    LIBRARY' 

<JF   THK 

UNIVERSITY  OF  CALIFORNIA. 

Received-- 


-.~ 

Accessions  No.....       Shelf  No. 


A  SYSTEM 

OF 

NATURAL  PHILOSOPHY: 

IN  WHICH  ARE   EXPLAINED  THE 

PRINCIPLES  OF  MECHANICS, 

HYDROSTATICS,    HYDRAULICS,   PNEUMATICS,   ACOUSTICS,   OPTICS, 

ASTRONOMY,    ELECTRICITY,    MAGNETISM,    STEAM-ENGINE, 

ELECTRO-MAGNETISM,  ELECTROTYPE,  PHOTOGRAPHY, 

AND  DAGUERREOTYPE; 

\ 

TO   WHICH   ARE    ADDED 

QUESTIONS   FORTHE  EXAMINATION  OF  PUPILS, 

DESIGNED    FOR 

THE  USE  OF  SCHOOLS  AND  ACADEMIE& 


ILLUSTRATKD      BY      THREE      HUNDRED      EN GRAVING!. 

~BY^L  L.  COMSTOCK;*M.  D. 

OT  INTRODUCTION  TO  MINERALOGY,  ELEMENTS  OP  CHEMISTRY,  IXTRODCCTTOIf  TO  BO 
OF  GEOLOGY,  OUTLINES  Of  PHYSIOLOGY,  R'ATTTRAL  HISTORY  OF  BIRDS,  CTC. 


ONE    BUVr-^-n    AND    SIXTIETH     EDITION. 
REVISED  AND  ENLARGED. 


PRATT,    WOODFORD,    AND    CO. 
1853. 


ENTERED,  according  to  Act  of  Congress,  in  the  year  of  our  Lord 
thousand  eight  hundred  and  fifty -two, 

By  J.  L.  COMSTOCK, 
In  the  Clerk's  Office  of  the  District  Court  of  Connecticut. 


STEREOTYPED   BY  PRINTED   BY 

RICHARD   H.    HOBBS,  CASE,  TIFFANY,   AND    CO., 

HARTFORD,   CONN.  HARTFORD,  CONN. 


NOTICE  BY  THE  PUBLISHERS 


THE  publishers  of  Dr.  Comstock's  Natural  Philosophy  will  not 
withhold  from  the  public  an  expression  of  the  gratification  they 
feel  as  Americans,  at  the  manner  in  which  the  work  has  been  re- 
ceived and  appreciated  in  Europe. 

It  has  been  twice  edited  and  republished  in  the  Queen's  do- 
minions. First  in  Scotland,  the  editor  being  Prof.  Lees  "of  the 
Naval  and  Military  Academy,  and  Lecturer  on  Natural  Philoso- 
phy, Edinburgh." 

In  his  Preface  the  editor  says:  "Among  the  many  works  on 
Natural  Philosophy  which  have  made  their  appearance  of  late 
years,  we  certainly  have  not  met  with  one  uniting  in  a  greater 
degree  the  two  grand  requisites  of  precision  and  simplicity  than 
Jie  work  of  Dr.  Comstock.  *  *  *  *  The  principle*  of  the 
science  are  stated  with  singular  clearness,  and  illustrated  by  the 
most  apt,  and  interesting  examples.  *  *  *  *  The  develop- 
ment of  the  various  branches  is  effected  by  the  help  of  well-de- 
signed diagrams,  and  these  by  no  means  sparingly  introduced. 
Published  by  Scott,  Webster,  $  Geary,  London. 

During  the  last  year  the  Philosophy  was  again  edited  by  Prof 
Hoblyn  of  Oxford,  now  "  Lecturer  in  the  Institute  of  Medicine 
and  Arts,"  London  ;  and  author  of  a  Medical  Dictionary  repub~ 
lished  in  this  country.  This  edition  is  dedicated  to  Marshall  Hall, 
M.  D.,  F.  R.  S.,  one  of  the  chiefs  of  the  Medical  profession  in 
the  metropolis,  and  who,  it  appears,  has  introduced  it  to  his  pupils 
in  the  lecture  room. 

The  following  is  an  extract  from  Prof.  Hoblyn's  Preface  to  his 
edition : — 

"  This  Manual  of  Natural  Philosophy  claims  no  higher  merit 
than  that  of  being  a  republication  of  the  popular  treatise  of  Dr. 
Comstock  of  Hartford,  in  the  United  States,  enlarged  and  to  a 
certain  extent  remodeled.  His  colleague  feels  a  peculiar  pleasure 


IV  NOTICE    BY    THE    PUBLISHERS. 

in  the  association  of  his  own  name  with  that  of  an  author,  who 
has  earned  a  well-merited  reputation  in  the  pursuit  of  physical 
science.  As  an  elementary  work,  requiring  for  its  perusal  no 
mathematical  attainment,  nor  indeed  any  previous  knowledge  of 
Natural  Philosophy,  it  is  at  once  simple,  intelligible,  and  in  most 
Darts  familiar."  Published  by  Adam  Scott,  Charter  House 
Square,  London. 

Besides  these  two  editions  of  the  entire  work,  Dr.  Comstock's 
Philosophy  has  been  published  in  parts,  in  the  form  of  scientific 
tracts,  at  a  shilling  each,  for  general  circulation  in  England.  We 
understand  also,  that  the  work  has  been  translated  into  German, 
for  the  use  of  the  public  schools  in  Prussia. 

Having  thus  undergone  the  critical  examination  of  two  Profes- 
sors of  high  attainments  abroad,  who  have  each  corrected  its 
errors,  and  added  to  its  pages,  and  of  whose  labors,  we  have  no 
doubt  the  author  has  availed  himself,  we  now  offer  this  revised 
edition  to  the  public,  with  renewed  confidence  in  its  correctness, 
as  well  as  its  adaptation  to  the  purpose  for  which  the  work  is 
intended. 

NEW  YORK. 


PREFACE 

TO   THE    FIFTH    STEREOTYPE    REVISION. 


THE  author  has  me  satisfaction  of  being  again  called  upon  by 
his  well-known,  and  enterprising  publishers  to  superintend  the 
proofs  of  a  new  set  of  stereotype  plates  for  this  work,  being  the 
fifth  which  its  circulation  has  required  in  this  country. 

In  this  improved  edition,  although  large  portions  of  the  contents 
have  been  re- written,  and  much  new  matter  added,  the  author 
has  been  careful  not  to  make  such  changes  as  to  render  it  unfit  to 
be  used  in  classes  with  the  last  editiorb- 

In  attempting  to  make  this  copy  more  complete  and  useful 
than  the  former  ones,  the  author  has  availed  himself  of  the  addi- 
tions and  corrections,  made  by  Professor  Lees  of  "  the  Naval  and 
Military  Academy  of  Edinburgh,"  and  of  those  of  Professor  Hob- 
lyn,  Lecturer  in  "  The  Institute  of  Medicine  and  Arts,"  London, 
these  two  gentleman  having  done  him  the  honor  to  associate 
their  names  with  that  of  the  author,  in  two  several  editions  of 
this  work. 

In  the  Chapters  on  the  Steam  Engine,  and  Astronomy,  it  was 
found  that  many  paragraphs  might  be  omitted  with  advantage 
to  the  essential  parts  of  the  subjects.  These  erasures  have 
been  replaced  by  a  description  of  the  Railroad  Locomotive,  and 
by  the  insertion  of  Tables  containing  the  names  of  the  new 
Asteroids,  and  of  those  of  their  discoverers,  with  the  date  of  each 
discovery. 

Among  the  additions  will  be  found  descriptions  and  illustrations 
cf  McCormick's  Reaper ;  Sharps'  Rifle  ;  Printing  Presses , 
House's  Printing  Telegraph;  Manufacture  of  Percussion  Caps; 
The  Organ ;  Monochord  ;  Hygrometer ;  Harmonicon ;  Air-Gun ; 
Dipping  Needle,  and  much  new  matter  on  Electro-Magnetism. 

The  questions,  in  this  edition,  are  numbered,  to  correspond 


Vi  PREFACE. 

with  those  of  the  paragraphs ;  and  nearly  all  the  old  cuts  have 
been  re-engraved,  and  about  fifty  new  ones  added. 

And,  finally,  perhaps  the  author  will  be  excused  for  adding, 
that  besides  the  circulation  of  this  work  in  England,  Scotland, 
and  Prussia,  there  have  been  printed,  and  sold  more  than  half 
a  million  copies  in  this  country. 

J.  L.  0. 

HARTFORD,  April,  1852, 


or  THE 

'UNIVERSITY 


%JF( 


NATURAL  PHILOSOPHY,  &c. 


CHAPTER   I. 

THE    PROPERTIES    OF    BODIES 

NATURAL  PHILOSOPHY,  or  the  Science  of  Nature,  has  for  its 
objects  the  investigation  of  the  properties  of  all  natural  bodies 
and  their  mutual  action  on  each  other.  The  term  Physics  has 
a  similar  meaning.  •* 

1.  A  BODY  is  any  substance  of  which  we  can  gain  a  knowl- 
edge by  our  senses.     Hence,  air,  water,  and  earth,  in  all  their 
modifications,  are  called  bodies. 

2.  There  are  certain  properties  which  are  common  to  all 
bodies.     These   are  called  the  essential  properties  of  bodies. 
They  are  Impenetrability,  Extension,  Figure,  Divisibility,  In- 
ertia, and  Attraction. 

3.  IMPENETRABILITY. — By  impenetrability,  is  meant  that  two 
bodies  can  not  occupy  the  same  space  at  the  same  time,  or,  that 
the  ultimate  particles  of  matter  can  not  be  penetrated.     Thus, 
if  a  vessel  be  exactly  filled  with  water,  and  a  stone  or  any  other 
substance  heavier  than  water,  be  dropped  into  it,  a  quantity  of 
water  will  overflow,  just  equal  to  the  size  of  the  heavy  body. 
This  shows  that  the  stone  only  separates  or  displaces  the  parti- 
cles of  water,  and  therefore  that  the  two  substances  can  not  ex- 
ist in  the  same  place  at  the  same  time.     If  a  glass  tube  open 
at  the  bottom,  and  closed  with  the  thumb  at  the  top,  be  pressed 
down  into  a  vessel  of  water,  the  liquid  will  not  rise  up  and  fill 
the  tube,  because  the  air  already  in  the  tube  resists  it ;  but  if 
the  thumb  be  removed,  so  that  the  air  can  pass  out,  the  water 
will  instantly  rise  as  high  on  the  inside  of  the  tube  as  it  is  on 

What  are  the  objects  of  natural  philosophy  7  1.  What  is  a  body?  2.  Mention 
several  bodies.  What  are  the  essential  properties  of  bodies  1  3.  What  is  meant  by 
impenetrability  ?  How  is  it  proved  that  air  and  water  are  impenetrable  7 


8  PROPERTIES    OF    BODIES. 

the  outside.     This  shows  that  the  air  is  impenetrable  to  the 
water. 

4.  If  a  nail  be  driven  into  a  board,  in  common  language,  it 
is  said  to  penetrate  the  wood,  but  in  the  language  of  philoso- 
phy, it  only  separates,  or  displaces  the  particles  of  the  wood. 
The  same  is  the  case,  if  the  nail  be  driven  into  a  piece  of  lead ; 
the  particles  of  the  lead  are  separated  from  each  other,  and 
crowded  together,  to  make  room  for  the  harder  body,  but  the 
particles  themselves  are  by  no  means  penetrated  by  the  nail. 

5.  When  a  piece  of  gold  is  dissolved  in  an,  acid,  the  parti- 
cles of  the  metal  are  divided,  or  separated  from  each  other,  and 
diffused  in  the  fluid,  but  the  particles  of  gold  are  supposed  still 
to  be  entire,  for  if  the  acid  be  removed,  we  obtain  the  gold 
again  in  its  solid  form,  just  as  though  its  particles  had  never 
been  separated. 

6.  EXTENSION. — Every  body,  however  small,  must  have  length, 
breadth,  and  thickness,  since  no  substance  can  exist  without  them. 
By  extension,  therefore,  is  only  meant  these  qualities.     Exten- 
sion has  no  respect  to  the  size,  or  shape  ofia  body. 

7.  The  size  and  shape  of  a  block  of  wood  a  foot  square  is 
quite  different  from  that  of  a  walking-stick.      But  they  both 
equally  possess  length,  breadth,  and  thickness,  since  the  stick 
might  be  cut  into  little  blocks,  exactly  resembling  in  shape  the 
large  one.     And  these  little  cubes  might  again  be  divided  until 
they  were  only  the  hundredth  part  of  an  inch  in  diameter,  and 
still  it  is  obvious  that  they  would  possess  length,  breadth,  and 
thickness,  for  they  could  yet  be  seen,  felt,  and  measured.  "  But 
suppose  each  of  these  little  blocks  to  be  again  divided  a  thou- 
sand times,  it  is  true  we  could  not  measure  them,  but  still  they 
would  possess  the  quality  of  extension,  as  really  as  they  did  be- 
fore division,  the  only  difference  being  in  respect  to  dimensions. 

8.  FIGURE  OR  FORM  is  the  result  of  extension,  for  we  can  not 
conceive  that  a  body  has  length  and  breadth,  without  its  also 
having  some  kind  of  figure,  however  irregular. 

9.  Some  solid  bodies  have  certain  or  determinate  forms 
which  are  produced  by  nature,  and  are  always  the  same, 
wherever  they  are  found.  Thus,  a  crystal  of  quartz  has  six 
sides,  while  a  garnet  has  twelve  sides,  these  numbers  being  in- 
variable. Some  solids  are  so  irregular,  that  they  can  not  be 

4.  When  a  nail  is  driven  into  a  board  or  piece  of  lead,  are  the  particles  of  these 
bodies  penetrated  or  separated  1  5.  Are  the  particles  of  gold  dissolved,  or  only  sep- 
arated by  the  acid  ?  6.  What  is  meant  by  extension  ?  7.  In  how  many  directions 
do  bodies  possess  extension  1  8.  Of  what  is  figure  or  form  the  result  1  Do  all 
bodies  possess  figure  7  9.  What  solids  are  regular  in  their  forms  7 


PROPERTIES    OF  BODIES.  9 

compared  with  any  mathematical  figure.  This  is  the  case 
with  the  fragments  of  a  broken  rock,  chips  of  wood,  fractured 
glass,  &c. ;  these  are  called  amorphous. 

10.  Fluid  bodies  have  no  determinate  forms,  but  take  their 
shapes  from  the  vessels  in  which  they  happen  to  be  placed. 

11.  DIVISIBILITY. — By  the  divisibility  of  matter,  we  mean 
that  a  body  may  be  divided  into  parts,  and  that  these  parts  may 
again  be  divided  into  other  parts. 

12.  It  is  quite  obvious,  that  if  -we  break  a  piece  of  marble 
into  two  parts,  these  two  parts  may  again  be  divided,  and  that 
the  process  of  division  may  be  continued  until  these  parts  are 
so  small  as  not  individually  to  be  seen  or  felt.     But  as  every 
body,  however  small,  must  possess  extension  and  form,  so  we 
can  conceive  of  none  so  minute  but  that  it  may  again  be  di- 
vided.     There   is,  however,  possibly  a   limit,  beyond   which 
bodies  can  not  actually  be  divided,  for  there  may  be  reason  to 
believe  that  the  atoms  of  matter  are  indivisible  by  any  means 
in  our  power.     But  under  what  circumstances  this  takes  place, 
or  whether  it  is  in  the  power  of  man  during  his  whole  life,  to 
pulverize  any  substance  so  finely,  that  it  may  not  again  be 
broken,  is  unknown. 

13.  We  can  conceive,  in  some  degree,  how  minute  must  be 
the  particles  of  matter,  from  circumstances  that  every  day  come 
within  our  knowledge. 

14.  A  single  grain  of  musk  will  scent  a  room  for  years,  and 
still  lose  no  appreciable  part  of  its  weight.     Here,  the  particles 
of  musk  must  be  floating  in  the  air  of  every  part  of  the  room, 
otherwise  they  could  not  be  every  where  perceived. 

15.  Gold  is  hammered  so  thin,  as  to  take  282,000  leaves  to 
make  an  inch  in  thickness.     Here,  the  particles  still  adhere 
to  each  other,  notwithstanding  the  great  surface  which  they 
cover, — a  single  grain  being  sufficient  to  extend  over  a  surface 
of  fifty  square  inches. 

16.  INDESTRUCTIBILITY. — This  term  means  that  nothing  is 
destroyed.     The  ultimate  particles  of  matter,  however  widely 
they  may  be  diffused,  are  not  individually  destroyed,  or  lost, 
but  under  certain  circumstances,  may  again  be  collected  into  a 
body  without  change  of  form.      Mercury,  water,  and  many 
other  substances,  may  be  converted  into  vapor,  or  distilled  in 

9.  What  bodies  are  irregular?  11.  What  is  meant  by  divisibility  of  matter?  Is 
there  any  limit  to  the  divisibility  of  matter  ?  12.  Are  the  atoms  of  matter  divisible  ? 
14.  What  examples  are  given  of  the  divisibility  of  matter  ?  15.  How  many  leaves  of 
gold  does  it  take  to  make  an  inch  in  thickness  1  How  many  square  inches  may  a 
grain  of  gold  be  made  to  cover  ?  16.  Under  what  circumstances  may  the  particles 
of  matter  again  be  collected  in  their  original  form  ? 

1* 


10  PROPERTIES    OF    BODIES. 

close  vessels,  without  any  of  their  particles  being  lost.  In  such 
cases,  there  is  no  decomposition  of  the  substances,  but  only  a 
change  of  form  by  the  heat,  and  hence  the  mercury  and  water 
assume  their  original  state  again  on  cooling. 

17.  Where  bodies  suffer  decomposition  or  decay,  their  ele- 
mentary particles,  in  like  manner,  are  neither  destroyed  nor  lost, 
but  only  enter  into  new  arrangements  or  combinations  with 
other  bodies. 

18.  When  a  piece  of  wood  is  heated  in  a  close  vessel,  such 
as  a  retort,  we  obtain  water,  an  acid,  several  kinds  of  gas,  and 
there  remains  a  black,  porous  substance,  called  charcoal.     The 
wood  is  thus  decomposed,  or  destroyed,  and  its  particles  take  a 
new  arrangement,  and  assume  new  forms,  but  that  nothing  is 
lost,  is  proved  by  the  fact  tha^  if  the  water,  acid,  gases,  and 
charcoal,  be  collected  and  weighed,  they  will  be  found  exactly 
as  heavy  as  the  wood  was  before  distillation. 

19.  Bones,  flesh,  or  any  other  animal  substance,  may  in  the 
sawie  manner  be  made  to  assume  new  forms,  without  losing  a 
particle  of  the  matter  which  they  originally  contained. 

20.  The  decay  of  animal  or  vegetable  bodies  in  the  open  air, 
or  in  the  ground,  is  only  a  process  by  which  the  particles  of 
which  they  were  composed,  change  their  places  and  assume 
new  forms. 

21.  The  decay  and  decomposition  of  animals  and  vegetables 
on  the  surface  of  the  earth  form  the  soil,  which  nourishes  the 
growth  of  vegetables ;  and  these,  in  their  turn,  form  the  nu- 
triment of  animals.     Thus  is  there  a  perpetual  change  from 
life  to  death,  and  from  death  to  life,  and  as  constant  a  succes- 
sion in  the  forms  and  places,  which  the  particles  of  matter  as- 
sume.    Nothing  is  lost,  and  not  a  particle  of  matter  is  struck 
out  of  existence.     The  same  matter  of  which  every  living  ani- 
mal, and  every  vegetable  was  formed  since  the  creation,  is  still 
in  existence.     As  nothing  is  lost  or  annihilated,  so  it  is  proba- 
ble that  nothing  has  been  added,  and  that  we,  ourselves,  are 
composed  of  particles  of  matter  as  old  as  the  creation.     In  time, 
we  must,  in  our  turn,  suffer  decomposition,  as  all  forms  have 
done  before  us,  and  thus  resign  the  matter  of  which  we  are 
composed,  to  form  new  existences. 

22.  INERTIA. — Inertia  means  pa-ssivcness  or  want  of  power 

17.  What  is  meant  by  indestructibility  ?  18.  When  bodies  suffer  decay,  are  their 
particles  lost?  19.  What  becomes  of  the  particles  of  bodies  which  decay  1  21.  Is  it 
probable  that  any  matter  has  been  annihilated  or  added,  since  the  first  creation? 
What  is  said  of  the  particles  of  matter  of  which  we  are  made? 


PROPERTIES    OF    BODIES.  11 

Thus,  matter  is,  of  itself,  equally  incapable  of  putting  itself  in 
motion,  or  of  bringing  itself  to  rest  when  in  motion. 

23.  It  is  plain  that  a  rock  on  the  surface  of  the  earth  never 
changes  its  position  in  respect  to  other  things  on  the  earth.     It 
has  of  itself  no  power  to  move,  and  would,  therefore,  forever  lie 
still,  unless  moved  by  some  external  force.     This  fact  is  proved 
by  the  experience  of  every  person,  for  we  see  the  same  objects 
lying  in  the  same  positions  all  our  lives.     Now,  it  is  just  as  true, 
that  inert  matter  has  no  power  to  bring  itself  to  rest,  when  once 
put  in  motion,  as  it  is  that  it  can  not  put  itself  in  motion  when 
at  rest,  for  having  no  life,  it  is  perfectly  passive,  both  to  motion 
and  rest ;  and  therefore  either  state  depends  entirely  upon  cir- 
cumstances. 

24.  Common  experience  proving  that  matter  does  not  put 
itself  in  motion,  we  might  be  led  to  believe,  that  rest  is  the  nat- 
ural state  of  all  inert  bodies  ;  but  a  few  considerations  will  show 
that  motion  is  as  much  the  natural  state  of  matter  as  rest,  and 
that  either  state  depends  on  the  resistance,  or  impulse,  of  ex- 
ternal causes. 

25.  If  a  cannon-ball  be  rolled  upon  the  ground,  it  will  soon 
cease  to  move,  because  the  ground  is  rough,  and  presents  imped- 
iments to  its  motion ;  but  if  it  be  rolled  on  the  ice,  its  motion 
will  continue  much  longer,  because  there  are  fewer  impediments, 
and  consequently,  the  same  force  of  impulse  will  carry  it  much 
further.      We  see  from  this,  that  with  the  same  impulse,  the 
distance  to  which  the  ball  will  move  must  depend  on  the  im- 
pediments it  meets  with,  or  the  resistance  it  has  to  overcome. 
But  suppose  that  the  ball  and  ice  were  both  so  smooth  as  to  re- 
move as  much  as  possible  the  resistance  caused  by  friction,  then 
it  is  obvious  that  the  ball  would  continue  to  move  longer,  and 
go  to  a  greater  distance.     Next,  suppose  we  avoid  the  friction 
of  the  ice,  and  throw  the  ball  through  the  air,  it  would  then 
continue  in  motion  still  longer  with  the  same  force  of  projection, 
because  the  air  alone  presents  less  impediment  than  the  air  and 
ice,  and  there  is  now  nothing  to  oppose  its  constant  motion,  ex- 
cept the  resistance  of  the  air,  and  its  own  weight,  or  gravity. 

26.  If  the  air  be  exhausted,  or  pumped  out  of  a  vessel  by 
means  of  an  air-pump,  and  a  common  top,  with  a  small,  hard 
point,  be  set  in  motion  in  it,  the  top  will  continue  to  spin  for 

22.  What  does  inertia  mean  1  24.  Is  rest  or  motion  the  natural  state  of  matter  ? 
25  Why  floes  the  ball  roll  farther  on  the  ice  than  on  the  ground  ?  What  does  this 
prove?  Why,  with  the  same  force  of  projection,  will  a  ball  move  further  through 
the  air  than  on  the  ice  ?  26.  Why  will  atop  spin,  or  a  pendulum  swing,  longer  inaa 
exhausted  vessel  than  in  the  u.ir  ? 


12  PROPERTIES    OF    BODIES. 

hours,  because  the  air  does  not  resist  its  motion.  A  pendulum, 
set  in  motion,  in  an  exhausted  vessel,  will  continue  to  swing, 
without  the  help  of  clock-work,  for  a  whole  day,  because  there 
is  nothing  to  resist  its  perpetual  motion  but  the  small  friction  at 
the  point  where  it  is  suspended,  and  gravity. 

27.  We  see,  then,  that  it  is  the  resistance  of  the  air,  of  fric- 
tion, and  of  gravity,  which  cause  bodies  once  in  motion  to 
'cease  moving,  or  come  to  rest,  and  that  dead  matter,  of  itself, 
is  equally  incapable  of  causing  its  own  motion,  or  its  own  rest. 

28.  We  have  perpetual  examples  of  the  truth  of  this  doc- 
trine, in  the  moon,  and  other  planets.     These  vast  bodies  move 
through  spaces  which  are  void  of  the  obstacles  of  air  and  fric- 
tion, and  their  motions  are  the  same  that  they  were  thousands 
of  years  ago,  or  at  the  beginning  of  creation. 

29.  ATTRACTION. — By  attraction  is  meant  that  property  or 
quality  in  the  particles  of  bodies,  which  makes  them  tend  toward 
each  other. 

30.  We  know  that  substances  are  composed  of  small  atoms 
or  particles  of  matter,  and  that  it  is  a  collection  of  these,  united 
together,  that  form   all  the   objects  with  which  we   are   ac- 
quainted.    Now,  when  we  come  to  divide,  or  separate  any  sub- 
stance into  parts,  we  do  not  find  that  its  particles  have  been 
united  or  kept  together  by  glue,  little  nails,  or  any  such  me- 
chanical means,  but  that  they  cling  together  by  some  power, 
not  obvious  to  our  senses.     This  power  we  call  Attraction,  but 
of  its  nature  or  cause,  we  are  entirely  ignorant.     Experiment 
and  observation,  however,  demonstrate  that  this  power  pervades 
all  material  things,  and  that  under  different  modifications,  it 
not  only  makes  the  particles  of  bodies  adhere  to  each  other,  but 
is  the  cause  which  keeps  the  planets  in  their  orbits  as  they  pass 
through  the  heavens. 

3 1 .  Attraction  has  received  different  names,  according  to  the 
circumstances  under  which  it  acts. 

32.  The  force  which  keeps  the  particles  of  matter  together, 
to  form  bodies,  or  masses,  is  called  Attraction  of  cohesion. 
That  which  inclines   different   masses   toward  each   other,  is 
called  Attraction  of  gravitation.     That  which  causes  liquids  to 
rise  in  tubes,  is  called  Capillary  attraction.     That  which  forces 

27.  What  are  the  causes  which  resist  the  perpetual  motion  of  bodies  ?  v  28.  Where 
have  we  an  example  of  continued  motion  without  the  existence  of  air  and  friction  1 
29.  What  is  meant  by  attraction  1  30.  What  is  known  about  the  cause  of  attraction  ? 
Is  attraction  common  to  all  kinds  of  matter,  or  not?  What  effect  does  this  power 
have  upon  the  planets'?  31.  Why  has  attraction  received  different  names?  32.  How 
many  kinds  of  attraction  are  there  1  How  does  the  attraction  of  cohesion  operate? 
What  is  meant  by  attraction  of  gravitation  ?  What  by  capillary  attraction  ? 


PROPERTIES    OF    BODIES.  13 

the  particles  of  substances  of  different  kinds  to  unite,  is  known 
under  the  name  of  Chemical  attraction.  That  which  causes 
the  needle  to  point  constantly  toward  the  poles  of  the  earth,  is 
Magnetic  attraction ;  and  that  which  is  excited  by  friction  in 
certain  substances,  is  known  by  the  name  of  Electrical  at- 
traction. 

33.  The  following  illustrations,  it  is  hoped,  will  make  each 
kind  of  attraction  distinct  and  obvious  to  the  mind  of  the  student. 

34.  ATTRACTION  OF  COHESION  acts  only  at  insensible  distances, 
as  when  the  particles  of  bodies  apparently  touch  each  other. 

35.  Take   two 

pieces  of  lead,  Fig.  FIG-  L 

1,  of  a  round  form, 
an  inch  in  diame- 
ter and  two  inches 
long  ;  flatten  one 
end  of  each  and  Cohesion. 

make1  through   it 

an  eye-hole  for  a  string.  Make  the  other  ends  of  each  as 
smooth  as  possible,  by 'cutting  them  with  a  sharp  knife.  If  now 
the  smooth  surfaces  be  brought  together,  with  a  slight  turning 
pressure,  they  will  adhere  with  such  force  that  two  men  can 
hardly  pull  them  apart  by  the  two  strings. 

36.  In  like  manner,  two  pieces  of  plate  glass,  when  their 
surfaces  are  cleaned  from  dust,  and  they  are  pressed  together, 
will  adhere  with  considerable  force.     Other  smooth  substances 
present  the  same  phenomena. 

37.  This  kind  of  attraction  is  much  stronger  in  some  bodies 
than  in  others.     Thus,  it  is  stronger  in  the  metals  than  in  most 
other  substances,  and  in  some  of  the  metals  it  is  stronger  than 
in  others.     In  general  it  is  most  powerful  among  the  particles 
of  solid  bodies,  weaker  among  those  of  liquids,  and  probably 
entirely  wanting  among  elastic  fluids,  such  as  air  and  the  gases. 

38.  Thus,  a  small  iron  wire  will  hold  a  suspended  weight 
of  many  pounds,  without  having  its  particles  separated ;  the 
particles  of  water  are  divided  by  a  very  small  force,  while  those 
of  air  are  still  more  easily  moved  among  each  other.     These 
different  properties  depend  on  the  force  of  cohesion  with  which 
the  several  particles  of  these  bodies  are  united. 

32.  What  by  chemical  attraction  ?  What  is  that  which  makes  the  needle  point 
toward  the  pole  ?  How  is  electrical  attraction  excited  ?  53.  Give  an  example  of  cohe- 
sive attraction  1  37.  In  what  substances  is  cohesive  attraction  the  strongest?  la 
what  substances  is  it  the  weakest }  38.  Why  are  the  particles  <>f  "aids  more  easily 
separated  than  those  of  solids  1 


14  PROPERTIES    OF    BODIES. 

39.  When  the  particles  of  fluids  are  left  to  arrange  them- 
selves according  to  the  laws  of  attraction,  the  bodies  which  they 
compose  assume  the  form  of  a  globe  or  ball. 

40.  Drops  of  water  thrown  on  an  oiled  surface,  or  on  wax, — 
globules  of  mercury, — hailstones, — a  drop  of  water  adhering  to 
the  end  of  the  finger, — tears  running  down  the  cheeks,  and  dew- 
drops  on  the  leaves  of  plants,  are  all  examples  of  this  law  of 
attraction.     The  manufacture  of  shot  is  also  a  striking  illustra- 
tion.    The  lead  is  melted  and  poured  into  a  sieve,  at  the  height 
of  about  two  hundred  feet  from  the  ground.     The  stream  of 
lead,  immediately  after  leaving  the  sieve,  separates  into  round 
globules,  which,  before  they  reach  the  ground,  are  cooled  and 
Income  solid,  and  thus  are  formed  the  shot  used  by  sports- 
men. « 

41.  To  account  for  the  globular  form  in  all  these  cases,  we 
have  only  to  consider  that  the  particles  of  matter  are  mutually 
attracted  toward  a  common  center,  and  in  liquids  being  free  to 
move,  they  arrange  themselves  accordingly. 

42.  In  all  figures  except  the  globe  or  ball,  some  of  the  parti- 
cles must  be  nearer  the  center  than  others.     But  in  a  body  that 
is  perfectly  round,  every  part  of  the  outside  is  exactly  at  the 
same  distance  from  the  center. 

43.  Thus,  the  corners  of  a  cube,  or 

square,  are  at  much  greater  distances  FIG.  2. 

from  the  center  than  the  sides,  while 
the  circumference  of  a  circle  or  ball  is 
every  where  at  the  same  distance  from 
it.  This  difference  is  shown  by  Fig.  2, 
and  it  is  quite  obvious,  that  if  the  parti- 
cles of  matter  are  equally  attracted  to- 
ward the  common  center,  and  are  free 
to  arrange  themselves,  no  other  figure 
could  possibly  be  formed,  since  then  Globular  form. 

every  part  of  the  outside  is  equally  at- 
tracted. 

44.  The  sun,  earth,  moon,  and  indeed  all  the  heaven  y  bodies, 
are  illustrations  of  this  law,  and  therefore  were  probably  in  so 
soft  a  state  when  first  formed,  as  to  allow  their  particles  freely 
to  arrange  themselves  accordingly. 

39.  What  form  do  fluids  take,  when  their  particles  are  left  to  their  own  arrange- 
ment ?  40.  Give  examples  of  this  law.  41.  How  is  the  globular  form  which  liquids 
assume  accounted  for  ?  If  the  particles  of  a  body  are  free  to  move,  and  are  equal- 
ly attracted  toward  the  center,  what  must  be  its  figure  ?  43.  Why  must  the  figure 
be  a  globe  7  44.  What  great  natural  bodies  are  examples  of  this  law  1 


PROPERTIES    OF    BODIES. 


15 


FIG.  3. 


45.  ADHESION. — The  attraction  between  solids  and  liquids 
is  termed  adhesion.     This  is  well  illustrated  by  means  of  Fi(j.  3. 

First,very  nice- 
ly balance  the 
plate  of  copper, 
C,~by  means  of 
weights  in  the 
cup,A,  and  then 
slide  the  vessel 
of  water,  B,  un- 
der the  copper, 
pouring  in  more 
of  the  fluid  un- 


Adhesion  between  solids  and  liquids. 


FIG.  4. 


til  the  metal  just 
touches  it.  Now 

on  placing  weights  in  A,  it  will  be  found  that  the  metal  ad- 
heres to  the  water  with  so  much  force,  that  -if  the  plate  has  an 
area  of  about  seven  inches,  it  will  require  a  weight  of  more  than 
1000  grains  to  raise  it  from  the  surface  of  the  water. 

46.  ATTRACTION  OF  GRAVITATION. — As    the   attraction    of 
cohesion  unites  the  particles  of  matter  into  masses  or  bodies,  so 
the  attraction  of  gravitation  tends  to  force  these  masses  toward 
each  other ,  to  form  those  of  still  greater  dimen- 
sions.    The   term  gravitation,  does   not   here 

strictly  refer  to  the  weight  of  bodies,  but  to 
the  attraction  of  the  masses  of  matter  toward 
each  other,  whether  downward,  upward,  or 
horizontally. 

47.  The  attraction  of  gravitation  is  mutual, 
since  all  bodies  not  only  attract  other  bodies, 
but  are  themselves  attracted. 

48.  Two  cannon-balls,  when  suspended  by 
long  cords,  so  as  to  hang  "quite  near  each  other, 
are  found  to  exert  a  mutual  attraction,  so  that 
neither  of  the  cords  are  exactly  perpendicular, 
but  they  approach  each  other  as  in  Fig.  4. 

49.  In  the  same  manner,  the  heavenly  bodies, 
when  they  approach  each  other,  are  drawn  out 
of  the  line  of  their  paths,  or  orbits,  by  mutual 

attraction.  Attraction. 

45.  What  explanation  can  yon  give  of  Fig.  3?  46.  What  is  meant  by  attraction  of 
gravitation?  47.  Can  one  body  attract  another  without  being  itself  attracted? 
43.  How  is  it  proved  that  bodies  attract  each  other  1 


16  PROPERTIES    OF   BODIES. 

50.  The  force  of  attraction  increases  in  proportion  as  bodies 
approach  each  other,  and  by  the  same  law  it  must  diminish  in 
proportion  as  they  recede  from  each  other.. 

51.  Attraction,  in   technical   language,  is  inversely  as  the 
squares  of  the  distances  between  the  two  bodies.     That  is,  in 
proportion  as  the  square  of  the  distance  increases,  in  the  same 
proportion  attraction  decreases,  and  so  the  contrary.     Thus, 
if  at  the  distance  of  2  feet,  the  attraction  be  equal  to  4  pounds, 
at  the  distance  of  4  feet,  it  will  be  only  1  pound;  for  the 
square  of  2  is  4,  and  the  square  of  4  is  16,  which  is  4  times 
the  square  of  2.     On  the  contrary,  if  the  attraction  at  the  dis- 
tance of  6  feet  be  3  pounds,  at  the  distance  of  2  feet  it  will  be 
9  times  as  much,  or  27  pounds,  because  36,  the  square  of  6, 
is  equal  to  9  times  4,  the  square  of  2. 

52.   The  law 

of    attraction   in  FIG.  5. 

masses  is  very  sat- 
isfactorily shown 
by  the  two  little 
cork  balls  in  Fig. 
5.  They  are  cover- 
ed with  Varnish,  Attraction  of  cork  balls. 

or  beeswax,  to  re- 
pel the  water. 

Two  such  balls  placed  on  the  surface  of  a  dish  of  water,  two 
or  three  inches  apart,  and  not  near  the  side  of  the  dish,  will 
soon  begin  to  approach  each  other;  their  velocities  being 
in  proportion  to  their  sizes,  and  increasing  as  their  distances  di- 
minish, until  quite  near,  when  they  rush  together  as  though 
they  had  life. 

53.  The  intensity  of  light  is  found  to  increase  and  diminish 
in  the  same  proportion.  Thus,  if  a  board  a  foot  square,  be 
placed  at  the  distance  of  one  foot*  from  a  candle,  it  will  be 
found  to  hide  the  light  from  another  board  of  two  feet  square, 
at  the  distance  of  two  feet  from  the  candle.  Now  a  board  of 
two  feet  square  is  just  four  times  as  large  as  one  of  one  foot 
square,  and  therefore  the  light  at  double  the  distance  being 
spread  over  four  times  the  surface,  has  only  one  fourth  the  in- 
tensity. 


50.  By  what  law  or  rule,  does  the  force  of  attraction  increase  ?  5t.  Give  an  exam, 
pie  of  this  rule.  52.  How  is  attraction  illustrated  by  Fig  5  ?  53.  How  is  it  shown 
that  the  intensity  of  light  increases  and  diminishes  in  the  same  proportion  as  the  at 
traction  of  matter  7 


PROPERTIES  OF  BODIES!  17 

54.  The  force  of  the  attraction  of  gravitation,  is  in  proportion 
to  the  quantity  of  matter  the  attracting  body  contains. 

55.  Some  bodies  of  the  same  bulk  contain  a  much  greater 
quantity  of  matter  than  others  :  thus,  a  piece  of  lead  contains 
about  twelve  times  as  much  matter  as  a  piece  of  cork  of  the 
same  dimensions,  and  therefore  a  piece  of  lead  of  any  given 
size,  and  a  piece  of  cork  twelve  times  as  large,  will  attract  each 
other  equally. 

56.  CAPILLARY   ATTRACTION. — The  force   by   which  small 
tubes,  or  porous  substances,  raise  liquids  above  their  levels,  is 
called  capillary  attraction. 

5*7.  If  a  small  glass  tube  be  placed  in  water,  the  water  on 
the  inside  will  be  raised  above  the  level  of  that  on  the  outside 
of  the  tube.  The  cause  of  this  seems  to  be  nothing  more  than 
the  ordinary  attraction  of  the  particles  of  matter  for  each  other. 
The  sides  of  a  small  orifice  are  so  near  each  other  as  to  attract 
the  particles  of  the  fluid  on  their  opposite  sides,  and  as  all  at- 
traction is  strongest  in  the  direction  of  the  greatest  quantity  of 
matter,  the  water  is  raised  upward,  or  in  the  direction  of  the 
length  of  the  tube.  On  the  outside  of  the  tube,  the  opposite 
surfaces,  it  is  obvious,  can  not  act  on  the  same  column  of  water, 
and  therefore~the  influence  of  attraction  is  here  hardly  percep- 
tible in  raising  the  fluid.  This  seems  to  be  the  reason  why  the 
fluid  rises  higher  on  the  inside  than  on  the  outside  of  the  tube. 

58.  Height  and  size  of  the  bore. — The 

height   to   which    the    fluid    will   rise,  PJQ  6 

seems  to  depend,  not  on  the  specific 
gravity  of  the  fluid,  but  on  the  size  of 
the  bore. 

59.  Thus,  if  the  four  glass  tubes, 
shown  by  Fig.  6,  are  respectively  the 

10th,  20th,  40th,  and  80th  of  an"  inch         

in  diameter,  then  the   height  of  the 

a    .,   .  T_       -11   i        •  ,T_    •  CapiSary  attraction. 

fluid  in  each  will  be  inversely  as  their 
several  diameters. 

60.  On  comparing  the  elevation  of  several  fluids  in  tubes  of 
the  same  diameter,  it  has  been  found  that  water  rose  more  than 
three  times  as  high  as  sulphuric  acid,  though  the  latter  is  nearly 

54.  Do  bodies  attract  in  proportion  to  bulk,  or  quantity  of  matter  ?    55.  What 
Would  be  the  difference  of  attraction  between  a  cubic  inch  of'lead.  aud  a  cubic  inch  of 
cork  ?    Why  would  there  be  so  much  difference  1    56.  What  is  meant  by  capillary     „ 
attraction  1    57.  How  is  this  kind  of  attraction  illustrated  with  glass  tubes  ?     Why 
does  the  water  rise  higher  in  the  tube  than  it  does  on  the  outside  1    58.  On  what        / 
does  the  height  of  the  fluid  in  capillary  tubes  depend  ?    59.  Explain  Fig.  6.    60.  What 
is  the  difference  iu  height  between  sulphuric  acid  and  water  7 


18  PROPERTIES    OF    BODIES. 

twice  as  heavy  as  the  former,  and  therefore  contains  a  propor- 
tionate quantity  of  attractive  matter.  The  cause  of  this  differ- 
ence is  unknown. 

61.  Prevents  evaporation. — It  is  very  remarkable  that  capil- 
lary attraction  prevents '  evaporation.     Thus,  fine  glass  tubes, 
open  at  both  ends,  and  containing  water,  were  exposed  to  the 
influence  of  the  sun,  in  the  open  air,  for  months,  without  losing 
the  least  portion  of  their  contents. 

62.  It  is  well  known  that  mercury  in  a  small  vertical  tube  is 
depressed  around  the  sides  next  to  the  glass ;  but  rises  in  the 
center,  forming  the  section  of  a  ball.     This  is  owing  to  the 
strong  attraction  the  particles  of  this  metal  have  for  each  other, 
while  they  appear  to  have  none  for  the  glass.     This  attraction 
is  beautifully  shown  by  the  little  bright  globules  which  mercu- 
ry forms  on  being,  thrown  on  a  smooth  surface. 

63.  A  great  variety  of  porous  substances  are  capable  of  ca- 
pillary attraction.     If  a  piece  of  sponge  or  a  lump  of  sugar  be 
placed  so  that  its  lowest  corner  touches  the  water,  the  fluid;;  wll 
rise  up  and  \?^lhj  whole  mass.     In  the  same  manner,  the 
wick  of  a  lamp  \v:fl  carry  up  the  oil  to  supply  the  flame,-fchough 
the  flame  is  sevejal  inches  above  the  level  of  the  oil.     K  the 
end  of  a  towel  haaMns  to  be  left  in  a  basin  of  water,  it  will 
empty  the  basin  dp^s  contents.     And  on  the  same  principle, 
when  a  dry  w#dge  of  wood  is  driven  into  the  crevice  of  a  rock, 
and  afterward  moistened  with  water,  as  when  the  rain  falls 
upon  it,  it  will  absorb  the  water,  swell,  and  sometimes  split  the 
rock.      In  Germany  mill-stone  quarries  are  worked  in   this 
manner. 

64.  CHEMICAL  ATTRACTION  takes  place  between  the  particles 
of  substances  of  different  kinds,  and  unites  them  into  one  com- 
pound. 

65.  This  species  of  attraction  takes  place  only  between  the 
particles  of  certain  substances,  and  is  not,  therefore,  a  universal 
property.     It  is  also  known  by  the  name  of  chemical  affinity, 
because  the  particles  of  substances  having  an  affinity  between 
them,  will  unite,  while  those  having  no  affinity  for  each  other 
do  not  readily  enter  into  union. 

66.  There  seems,  indeed,  in^s  respect,  to  be  very  singular 
preferences,  and  dislikes,  existing  among  the  particles  of  matter. 

61.  What  is  said  of  its  preventing  evaporation?  62.  Why  does  mercury  form  a 
section  of  a  ball  in  a  glass  tube  ?  64.  What  is  the  effect  of  chemical  attraction  ? 
65.  By  what  other  name  is  this  kind  of  attraction  known  ?  66.  What  effect  is  pro- 
duced when  marble  and  sulphuric  acid  are  brought  together?  What  is  the  effect 
when  glass  and  this  acid  are  brought  together  7  What  is  the  reason  of  this  difference  ? 


PROPERTIES    OF    BODIES.  19 

Thus,  if  a  piece  of  marble  be  thrown  into  sulphuric  acid,  their 
particles  will  unite  with  great  rapidity  and  commotion,  and  there 
will  result  a  compound  differing  in  all  respects  from  the  acid 
or  the  marble.  But  if  a  piece  of  glass,  quartz,  gold,  or  silver, 
be  thrown  into  this  acid,  no  change  is  produced  on  either,  be- 
cause their  particles  have  no  affinity. 

67.  Sulphur  and  quicksilver,  when  heated  together,  will  form 
a  beautiful  red  compound,  known  under  the  name  of  vermilion, 
and  which  has  none  of  the  qualities  of  sulphur  or  quicksilver. 

68.  Oil  and  water  have  no  affinity  for  each  other,  but  pot- 
ash has  an  attraction  for  both,  and  therefore  oil  and  water  will 
unite  when  potash  is  mixed  with  them.     In  this  manner,  the 
well-known  article  called  soap  is  formed.     But  the  potash  has 
a  stronger  attraction  for  an  acid  than  it  has  for  either  the  oil  or 
the  water  ;  and  therefore,  when  soap  is  mixed  with  an  acid,  the 
potash  leaves  the  oil,  and  unites  with  the  acid,  thus  destroying 
the  old  compound,  and  at  the  same  instant  forming  a  new  one. 
liic  same  happens  when  soap  is  dissolved  in  any  water  con- 

,  tainmg  an  acid,  as  the  waters  of  the  seamaid  of  certain  wells. 

.,-The  potash  forsakes  the  oil,  and  unites  with  fRe  acid,  thus  leav- 
ing the  oil  to  rise  to  the  surface  of  the  water.  %  Such  waters  are 
called  hard,  and  will  not  wash,  becauseflfc|  acid  renders  the 
potash  a  neutral  substance.  ^H 

69.  MAGNETIC  ATTRACTION. — There  is  a  certain  ore  of  iron, 
a  piece  of  which,  being  suspended  by  a  thread,  will  always  turn 
one  of  its  sides  to  the  north.     This  is  called  the  loadstone,  or 
natural  magnet,  and  wj^en  it  is  brought  near  a  piece  of  iron,  or 
steel,  a  mutual  attraction  takes  place,  and  under  certain  circum- 
stances the  two  bodies  will  come  together,  and  adhere  to  each 
other.     This  is  called  Magnetic  Attraction.     When  a  piece  of 
steel  or  iron  is  rubbed  with  a  magnet,  the  same  virtue  is  com- 
municated to  the  steel,  and  it  will  attract  other  pieces  of  steel, 
and  if  suspended  by  a  string,  one  of  its  ends  will  constantly 
point  toward  the  north,  while   the   other,  of  course,  points 
toward  the  south.    This  is  called  an  artificial  magnet.    The  mag- 
netic needle  is  a  piece  of  steel,  first  touched  with  the  loadstone, 
and  then  suspended,  so  as  to  turn  easily  on  a  point.     By  means 
of  this  instrument,  the  mariner  guides  his  ship  through  the 
pathless  ocean.     See  Magnetism. 

67.  When  sulphur  and  quicksilver  are  combined,  what  is  formed  ?  68.  How  may 
oil  and  water  be  made  to  unite  7  What  is  the  composition  thus  formed  called  J 
How  does  au  acid  destroy  this  compound  1  What  is  the  reason  that  hard  water  will 
not  wash  ?  69.  What  is  a  natural  magnet  1  What  is  meant  by  magnetic  attraction  7 
What  is  an  artificial  magnet  1  What  is  a  magnetic  needle  1  What  is  its  use  1 


20  PROPERTIES    OF    BODIES. 

70.  ELECTRICAL  ATTRACTION. — When  a  piece  of  glass,  01 
sealing-wax,  is  rubbed  with  the  dry  hand,  or  a  piece  of  cloth, 
and  then  held  toward  any  light  substance,  such  as  hair  or 
thread,  the  light  body  will  be  attracted  by  it,  and  will  adhere 
for  a  moment  to  the  glass  or  wax.     The  influence  which  thus 
moves  the  light  body  is  called  Electrical  Attraction.     When 

.  the  light  body  has  adhered  to  the  surface  of  the  glass  for  a  mo- 
ment, it  is  again  thrown  off,  or  repelled,  and  this  is  called  Elec- 
trical Repulsion.  See  Electricity. 

71.  We  have  thus  described  and  illustrated  all  the  universal 
or  inherent  properties  of  bodies,  and  have  also  noticed  the  seve- 
ral kinds  of  attraction  which  are  peculiar,  namely,  Chemical, 
Magnetic,  and  Electrical.     There  are  still  several  properties  to 
be  mentioned.     Some  of  them  belong  to  certain  kinds  of  mat- 
ter in  a  peculiar  degree,  while  other  kinds  possess  them  but 
slightly,  or  not  at  all.     These  properties  are  as  follows : 

72.  DENSITY. — This  property  relates  to  the  compactness  of  . 
bodies,  or  the  number  of  particles  which  a  body  contains  within 
a  given  bulk.     It  is  closeness  of  texture. 

73.  Bodies  which  are  most  dense,  are  those  which  contain 
the  least  number  of  pores.     Hence,  the  density  of  the  metals  is 
much  greater  than  that  of  wood.     Two  bodies  being  of  equal 
bulk,  that  which  weighs  most  is  most  dense.     Some  of  the 
metals  may  have   this  quality  increased  by  hammering,  by 
which  their  pores  are  filled  up,  and  their  particles  are  brought 
nearer  to  each  other.     The  density  of  air  is  increased  by  forcing 
more  into  a  close  vessel  than  it  naturallv  contained. 

74.  RARITY. — This  is  the  quality  opposite  to  density,  and 
means  that  the  substance  to  which  it  is  applied  is  porous  and 
light.     Thus,  air,  water,  and  ether  are  rare  substances,  while 
gold,  lead,  and  platina  are  dense  bodies. 

75.  HARDNESS. — This  property  is  not   in  proportion,  as 
might  be  expected,  to  the  density  of  the  substance,  but  to  the 
force  with  which  the  particles  of  a  body  cohere,  or  keep  their 
places. 

76.  Glass,  for  instance,  will  scratch  gold  or  platina,  though 
these  metals  are  much  more  dense  than  glass.     It  is  probable, 
therefore,  that  these  metals  contain  the  greatest  number  of  par- 
ticles, )>ut  that  those  of  the  glass  are  more  firmly  fixed  in  their 
places. 

70.  What  is  meant  by  electrical  attraction  7  What  is  electrical  repulsion  1  71.  What 
properties  of  bodies  have  been  described  7  72.  What  is  density  7  73.  What  bodies 
are  most  dense  7  How  may  this  quality  be  increased  in  metals  7  74.  What,  is  rarity  7 
What  are  rare  bodies  7  What  are  dense  bodies  7  75.  How  does  hardness  differ  from 
density  7  76.  Why  will  glass  scratch  gold  or  platiua  7 


PROPERTIES    OF    BOD] 

P.3IT1 

77.  Some  of  the  metals  can  be  made  hard  or  soft  at  pleasure. 
Thus  steel,  when  heated,  and  then  suddenly  cooled,  becomes 
harder  than  glass ;  while,  if  allowed  to  cool  slowly^t'lsioft,  and 
flexible. 

78.  ELASTICITY  is  that  property  in  bodies  by  which,  after 
being  forcibly  compressed,  or  bent,  they  regain  tfieir  original 
state  when  the  force  is  removed. 

79.  Some  substances  are  highly  elastic,  while  others  want 
this  property  entirely.     The  separation  of  two  bodies  after  im- 
pact, is  a  proof  that  one  or  both  are  elastic.     In  general,  most 
hard  and  dense  bodies  possess  this  quality  in  greater  or  less  de- 
gree.    Ivory,  glass,  marble,  flint,  and  ice,  are  elastic  -solids. 
An  ivory  ball,  dropped  upon  a  marble  slab,  will  bound  nearly 
to  the  height  from  which  it  fell,  and  no  mark  will  be  left  on 
either.     India  rubber  is  exceedingly  elastic,  and,  on  being  thrown 
forcibly  against  a  hard  body,  will  bound  to  an  amazing  distance. 
Steel,  when  hardened  in  a  particular  manner,  and  wrought  into 
certain  forms,  possesses  this  property  in  the  highest  degree. 
Watch-springs,  and  those  of  carriages,  as  well  as  sword-blades, 
are  examples.     Gold,  silver,  copper,  and  platina,  also  have  this 
property  in  a  degree. 

80.  Putty,  dough,  and  wet  clay  are  examples  of  the  entire 
want  of  elasticity ;  and  if  either  of  these  be  thrown  against  an 
impediment,  they  will  be  flattened,  stick  to  the  place  they  touch, 
and  never,  like  elastic  bodies,  regain  their  former  shapes. 

81.  Among  fluids,  water,  oil,  and  in  general  all  such  substances 
as  are  denominated  liquids,  are  nearly  inelastic,  while  air,  and 
the  gaseous  fluids,  are  the  most  elastic  of  all  bodies. 

82.  Change  of  Form. — The  change  of 

form  in  an  elastic  body,  as  an  India  rubber  FIQ  7- 

ball,  is  shown  by  Fig.  7,  where  its  side, 

on  striking  an  impediment,  is  compressed 

to  a,  but  instantly  springs  to  b  ;  the  dark 

line  between  them  being  the  surface  in 

the  natural  state. 

83.  BRITTLENESS  is  the  property  which 
renders  substances  easily  broken,  or  sepa- 
rated into   irregular  fragments.      This  changehof form, 
property  belongs  chiefly  to  hard  bodies. 

84.  It  does  not  appear  that  brittleness  is  entirely  opposed  to 

77.  What  metal  can  be  made  hard  or  soft  at  pleasure  ?  78.  What  is  meant  by 
elasticity?  79.  How  is  it  known  that  bodies  possess  this  property  ?  Mention  seve- 
ral elastic  solids.  80.  Give  examples  of  inelastic  solids.  81.  Do  liquids  possess  this 
property  ?  What  are  the  most  elastic  of  all  substances?  82.  Explain  Fig.  7.  83.  What 
is  brittleness? 


22  PROPERTIES    OF    BODIES. 

elasticity,  since,  in  many  substances,  both  these  properties  are 
united.  Glass  is  the  standard,  or  type  of  brittleness ;  and  yet 
a  ball,  or  fine  threads  of  this  substance,  are  highly  elastic,  as 
may  be  seen  by  the  bounding  of  the  one,  and  the  springing  of 
the  other.  Brittleness  often  results  from  the  treatment  to 
which  substances  are  submitted.  Iron,  steel,  brass,  and  copper, 
become  brittle  when  heated  and  suddenly  cooled ;  but  if  cooled 
slowly,  they  are  not  easily  broken, 

85.  MALLEABILITY. —  Capability  of  being  drawn  under  the 
hammer  or  rolling-press. 

This  property  belongs  to  some  of  the  metals,  but  not  to  all, 
and  is  of  vast  importance  to  the  arts  and  conveniences  of  life. 

86.  The  malleable  metals  are  platina,  gold,  silver,  iron,  cop- 
per, lead,  tin,  and  some  others.     Antimony,  bismuth,  and  co- 
balt, are  brittle  metals.     Brittleness  is,  therefore,  the  opposite 
of  malleability. 

87.  Gold  is  the  most  malleable  of  all  substances.     It  may 
be  drawn  under  the  hammer  so  thin  that  light  may  be  seen 
through  it.     Copper  and  silver  are  also  exceedingly  malleable. 

88.  DUCTILITY  is  that  property  in  substances  which  renders 
them  susceptible  of  being  drawn  into  wire. 

89.  We  should  expect  that  the  most  malleable  metals  would 
also  be  the  most  ductile ;  but  experiment  proves  that  this  is 
not  the  case.     Thus,  tin  and  lead  may  be  drawn  into  thin 
leaves,  but  can  not  be  drawn  into  small  wire.     Gold  is  the  most 
malleable  of  all  the  metals,  but  platina  is  the  most  ductile. 
Dr.  Wollaston  drew  platina  into  threads  not  much  larger  than 
a  spider's  web. 

90.  TENACITY,  in  common  language  called  toughness,  refers 
to  the  force  of  cohesion  among  the  particles  of  bodies. 

Tenacious  bodies  are  not  easily  pulled  apart.  There  is  a  re- 
markable difference  in  the  tenacity  of  different  substances. 
Some  possess  this  property  in  a  surprising  degree,  while  others 
are  torn  asunder  by  the  smallest  force. 

91.  Tenacity  of  Wood. — The  following  is  a  tabular  view  of 
the  absolute  cohesion  of  the^  principal  kinds  of  timber  employed 
in  the  arts  and  in  building,  showing  the  weight  which  would 
rend  a  rod  an  inch  square,  and  also  the  length  of  the  rod, 
which,  if  suspended,  would  be  torn  asunder  by  its  own  weight. 

84.  Are  brittleness  and  elasticity  ever  found  in  the  same  substance  ?  Give  exam- 
ples. How  are  iron,  steel,  and  brass  made  brittle?  85.  What  does  malleability 
mean!  86.  What  metals  are  malleable,  and  what  are  brittle?  87.  Which  is  the 
most  malleable  metal  ?  88.  What  is  meant  by  ductility  ?  89.  Are  the  most  mallea- 
ble metals  the  most  ductile  ?  90.  What  is  meant  by  tenacity  1  From  what  does  this 
property  arise  7 


PROPERTIES    OF  BODIES.  23 

92.  It  appears,  by  experiment,  that  the  following  is  the  ave- 
rage tenacity  of  the  kinds  of  woods  named ;  but  it  is  found  that 
there  is  much  difference  in  the  strength  of  the  same  species, 
and  even  of  the  different  parts  of  the  same  tree. 

93.  The  first  line  refers  to  the  weight,  and  the  other  to  the 
length,  the  wood  being  an  inch  square. 

Pounds.  Feet. 

Teak,  ....  12,915 36,049 

Oak,   ....  11,880 32,900 

Sycamore, .     .     .  9,630 35,800 

Beech,  ....  12,225 38,940 

Ash,      ....  14,130 39,050 

Elm,      ....  9,540 40,500 

Larch,  ....  12,240 42,160 

94.  Tenacity  of  the  Metals.— The  metals  differ  much  more 
widely  in  their  tenacity  than  the  woods.     According  to  the  ex- 
periments of  Mr.  Rennie,  the  cohesive  power  of  the  several 
metals  named  below,  each  an  inch  square,  is  equal  to  the  num- 
ber of  pounds  marked  in  the  table,  while  the  feet  indicate  the 
length  required  to  separate  each  metal  by  its  own  weight. 

Pounds.  Feet. 

Cast  steel,     .  .  134,256 39,455 

Malleable  iron,  .  .  72,064 19,740 

Cast  iron,      .  .  .19,096 6,110 

Yellow  brass,  .  .17,958 5,180 

Cast  copper,  .  .  .  19,072 5,093 

Cast  tin,        .  .  .    4,736 1,496 

Cast  lead,      .  .  .    1,824 348 

The  cohesion  of  fluids  it  is  difficult  to  measure,  though  some 
indication  of  this  property  is  derived  by  the  different  sizes  of 
the  drops  of  each  on  a  plane  surface.  ' 

95.  RECAPITULATION. — The  common  or  essential  properties 
of  bodies  are,  Impenetrability,  Extension,  Figure,  Divisibility,  In- 
ertia, and  Attraction.  Attraction  is  of  several  kinds,  viz.  attraction 
of  Cohesion,  attraction  of  Gravitation,  Capillary  attraction,  Chem- 
ical attraction,  Magnetic  attraction,  and  Electrical  attraction. 

96.  The  peculiar  properties  of  bodies  are,  Density,  Rarity,  Hard- 
ness. Elasticity,  Brittleness,  Malleability,  Ductility,  and  Tenacity. 

93.  Give  the  names  of  the  most  tenacious  sorts  of  wood.  94.  What  metals  are  most 
tenacious  7  What  metals  are  least  tenacious  ?  95.  What  are  the  essential  properties 
of  bodies  ?  How  many  kinds  of  attraction  are  there  ?  96.  What  are  the  peculiaj 
properties  of  bodies  1 


CHAPTER  II. 

GRAVITY. 

97.  THE  force  by  which  bodies  are  drawn  toward  each  other 
in  the  mass,  and  by  which  they  descend  toward  the  earth  when 
let  fall  from  a  height,  is  called  the  force  of  gravity. 

98.  The  attraction  which   the  earth   exerts   on  all  bodies 
near  its  surface,  is  called    terrestrial  gravity  /  and  the  force 
with  which  any  substance  is  drawn  downward,  is  called  its 
weight. 

99.  All  falling  bodies  tend  downward,  or  toward  the  center 
of  the  earth,  in  a  straight  line  from  the  point  where  they  are 
let  fall.     If,  then,  a  body  descends,  in  any  part  of  the  world, 
the  line  of  its  direction  will  be  perpendicular  to  the  earth's  sur- 
face.    It  follows,  therefore,  that  two  falling  bodies,  on  opposite 
parts  of  the  earth,  mutually  fall  toward  each  other. 

100.  Suppose  a  cannon-ball  to  be  disengaged  from  a  height 
opposite  to  us,  on  the  other  side  of  the  earth,  its  motion  in  re- 
spect to  us  would  be  upward,  while  the  downward  motion  from 
where  we  stand  would  be  upward  in  respect  to  those  who  stand 
opposite  to  us  on  the  other  side  of  the  earth.  * 

101.  In  like  manner,  if  the  falling  body  be  a  quarter,  in- 
stead of  half  the  distance  round  the  earth  from  us,  its  line  of 
direction  will  be  directly  across,  or  at  right-angles  with  the  line 
already  supposed. 

102.  This  will  be  readily  understood  by  Fig.  8,  where  the 
circle  is  supposed  to  be  the  circumference  of  the  earth,  A,  the 
ball  falling  toward  its  upper  surface,  where  we  stand ;  B,  a  ball 
falling  toward  the  opposite  side  of  the  earth,  but  ascending  in 
respect  to  us ;  and  D,  a  ball  descending  at  the  distance  of  a 
quarter  of  the  circle  from  the  other  two,  and  crossing  the  liue 
of  their  direction  at  right-angles. 

103.  It  will  be  obvious,  therefore,  that  what  we  call  up  and 
down,  are  merely  relative  terms ;  and  that  what  is  down  in  re- 

97.  What  is  gravity  ?  98.  What  is  terrestrial  gravity  1  99.  To  what  point  in  the 
earth  do  falling  bodies  tend?  100.  In  what  direction  will  two  falling  bodies,  from 
opposite  parts  of  the  earth,  tend  in  respect  to  each  other  1  101.  In  what  direction 
will  one  from  half-way  between  them  meet  their  line  ?  102.  How  is  this  shown  by 
Fig.  8  ?  103.  Are  the  terms  up  and  down  relative  or  positive  in  their  meaning  ? 


GRAVITY. 


25 


Direction  of  Falling  Bodies. 


spect  to  us,  is  up  in  respect 
to  those  who  live  on  the 
opposite  side  of  the  earth, 
and  so  the  contrary.  Conse- 
quently, down  every  where 
means  toward  the  center  of 
the  earth ;  and  up,  from  the 
center  of  the  earth, "because 
all  bodies  descend  toward  the 
earth's  center  from  whatever 
part  they  are  let  fall.  This 
will  be  apparent  when  we 
consider  that,  as  the  earth 
turns  over  every  24  hours, 
we  are  carried  with  it  through 
the  points  A,  D,  and  B,  Fig. 
8 ;  and,  therefore,  if  a  body 
is  supposed  to  fall  from  the 
point  A,  say  at  12  o'clock,  and  the  same  to  foil  again  from  the 
same  point  above  the  earth  at  6  o'clock,  the  two  lines  of  directi  >n 
will  be  at  right-angles,  as  represented  in  the  figure,  for  that  part 
of  the  earth  which  was  under  A  at  12  o'clock,  will  be  under  D  at 
6  o'clock,  the  earth  having  in  that  time  performed  one  quarter 
of  its  daily  revolution.  At  12  o'clock  at  night,  if  the  body  be 
supposed  to  fiill  again,  its  line  of  direction  will  be  at  right-a  i- 
gles  with  that  of  its  last  descent,  and  consequently,  it  will  as- 
cend in  respect  to  the  point  from  which  it  fell  12  hours  before, 
because  the  earth  would  have  then  gone  through  one  half  her 
daily  rotation,  and  the  point  A  would  be  at  B. 

VELOCITY    OF    FALLING    BODIES. 

104.  The  velocity  of  every  falling  body  is  uniformly  accele- 
rated in  its  approach  toward  the  earth,  from  whatever  height  it 
falls. 

105.  If  a  rock  is  rolled  from  a  steep  mountain,  its  motion  is 
at  first  slow  and  gentle  ;  but,  as  it  proceeds  downward,  it  moves 
with  perpetually  increased  velocity,  seeming  to  gather  frjsh 
speed  every  moment,  until  its  force  is  such  that  every  obstacle 
is  overcome. 

106.  The  principle  of  increased  velocity  as  bodies  descend 

What  is  understood  by  dmcn  in  any  part  of  the  earth  ?  Suppose  a  ball  be  let  all 
at  12  and  then  at  6  o'clock,  in  what  direction  would  the  lines  of  their  descent  meet 
each  other  1  104.  What  is  said  concerning  the  motions  of  falling  bodies  7  105.  How 
is  this  increased  velocity  illustrated  ?  106.  Explain  Fig.  9 

o 


26 


GRAVITY. 


from  a  height,  is  curiously  illustrated  by 
pouring  molasses  or  thick  syrup  from  an  F1G  9- 

elevation  to  the  ground.  The  bulky  stream, 
Fig.  9,  of  perhaps  two  inches  in  diameter 
where  it  leaves  the  vessel,  as  it  descends,  is 
reduced  to  the  size  of  a  straw,  or  knitting- 
needle  ;  but  what  it  wants  in  bulk  is  made 
up  in  velocity,  for  the  small  stream  at  the 
ground  will  fill  a  vessel  just  as  soon  as  the 
large  one  at  the  outlet. 

107.  For  the  same  reason,  a  man  may 
leap  from  a  chair  without  danger,  but  if  he 
jumps  from  the  house-top,  his  velocity  be- 
comes so  much  increased  before  he  reaches 
the  ground,  as  to  endanger  his  life  by  the 
blow. 

It  is  found,  by  experiment,  that  the  mo- 
tion of  a  falling  body  is  accelerated  in  regu- 
lar mathematical  proportions. 

These  increased  proportions  do  not  de- 
pend on  the  increased  weight  of  the  body,         increased  Velocity. 
because  it  approaches  nearer  the  center  of 
the  earth,  but  on  the  constant  operation  of  the  force  of  gravity, 
which  perpetually  gives  new  impulses  to  the  falling  body,  and 
increases  its  velocity. 

108.  It  has  been  ascertained,  by  experiment,  that  a  body 
falling  freely,  and  without  resistance,  passes  through  a  space 
of  16  feet  and  1  inch  during  the  first  second  of  time.     Leaving 
out  the  inch,  which  is  not  necessary  for  our  present  purpose, 
the  ratio  of  descent  is  as  follows  : 

109.  If  the  height  through  which  a  body  falls  in  one  second 
of  time  be  known,  the  height  which  it  falls  in  any  proposed 
time  may  be  computed.     For  since  the  height  is  proportional 
to  the  square  of  the  time,  the  height  through  which  it  will  fall 
in  two  seconds  will  be  four  times  that  which  it  falls  through  in 
one  second.     In  three  seconds  it  will  fall  through  nine  times  that 
space  ;  in  four  seconds  sixteen  times  that  of  the  first  second  ;  in 
five  seconds  twenty-five  times,  and  so  on  in  this  proportion. 

The  following,  therefore,  is  a  general  rule  to  find  the  height 
through  which  a  body  will  fall  in  any  given  time. 


107.  Why  is  there  more  danger  in  jumping  from  the  house-top  than  from  a  chair  ? 

108.  What    number  of  feet  does  a  falling  body  pass  through  in  the  first  second? 

109.  If  a  body  fall  from  a  certain  height  in  two  seconds,  what  proportion  to  this  will 
it  fall  in  four  seconds  1 


GRAVITY. 


27 


110.  Rule. — Reduce  the  given  time  to   seconds ;    take  the 
square  of  ihe  number  of  seconds  in  the  time,  and  multiply  the 
height  through  which  the  body  falls  inone  second  by  that  num- 
ber, and  the  result  will  be  the  height  sought. 

111.  The  following  table  exhibits  the  height  in  feet,  and  the 
corresponding  times  in  seconds. 


Time 
Height 

1 
1 

2 
4 

3 
9 

4 

16 

5 

25 

6 
36 

7 
49 

8 
64 

9 
81 

10 
100 

Now,  as  the  body  falls  at  the  rate  of  16  feet  during  the  first 
second,  this  number,  according  to  the  rule,  multiplied  by  the 
square  of  the  time,  that  is,  by  the  numbers  expressed  in  the  sec- 
ond line,  will  show  the  actual  distance  through  wrhich  the  body 
falls. 

112.  Thus  we  have  for  the  first  second  16  feet ;  for  the  end 
of  the  second  ;   4  X  16  =  64  feet ;  third,  9  X  16  =  144  ;  fourth, 
16x16  =  256;    fifth, .  25  X  16  =  400  ;    sixth,   36x16  =  576; 
seventh,  49  x  16  =  784  ;  and  for  the  10  seconds  1600  feet 

113.  If,  on  dropping  a  stone  from  a  precipice,  or  into  a  well, 
we  count  the  seconds  from  the  instant  of  letting  it  fall  until  we 
hear  it  strike,  we  may  readily  estimate  the  height  of  the  preci- 
pice, or  the  depth  of  the  well.     Thus,  suppose  it  is  5  seconds  in 
falling,  then  we  only  have  to  square  the  seconds,  and  multiply 
this  by  the  distance  the  body  falls  in  one  second.     We  have 
then  5  X  5  =  25,  the  square,  which  25  X  16  =  400  feet,  the  depth 
of  the  well. 

114.  Thus  it  appears,  that  to  ascertain   the  velocity  with 
which  a  body  falls  in  any  given  time,  we  must  know  how  many 
feet  it  fell  during  the  first  second :  the  velocity  acquired  in  one 
second,  and  the  space  fallen  through  during  that  time,  being  the 
fundamental  elements  of  the  whole  calculation,  and  all  that  are 
necessary  for  the  computation  of  the  various  circumstances  of 
falling  bodies. 

115.  The  difficulty  of  calculating  exactly  the  velocity  of  a 
falling  body  from  actual  measurement  of  its  height,  and  the 
time  which  it  takes  to  reach  the  ground,  is  so  great,  that  no  ac- 
curate computation  could  be  made  from  such  an  experiment. 

116.  ATWOOD'S  MACHINE. — This  difficulty  has,  however,  been 
overcome  by  a  curious  piece  of  machinery  invented  by  Mr.  At- 
wood.     This  consists  of  an  upright  pillar,  with  a  wheel  on  the 

110.  What  is  the  rule  by  which  the  height  from  which  a  body  falls  may  be  found  ? 
112.  How  many  feet  will  a  body  fall  in  10  stco  ids  I  113.  If  the  stone  is  5  seconds  in 
falling,  how  de'ep  is  the  well  1  116.  Is  the  velocity  of  a  falling  body  calculated  from 
actual  measurement,  or  by  a  machine  ? 


28 


GRAVITY. 


no. 


top,  as  shown  by  Fig.  10.  The 
weights  A  and  B  are  of  the  same 
size,  and  are  made  to  balance  each 
other  exactly,  being  connected  by 
a  thread  passing  over  the  wheel. 
The  ring,  R,  admits  the  weight,  A, 
to  fall  through  it  in  its  passage  to 
the  stage,  S,  on  which  it  rests. 
The  ring  and  stage  slide  lip  and 
down,  and  are  fastened  by  a  thumb- 
screw. The  pillar  is  a  graduated 
scale,  and  M  is  a  small  bent  wire, 
weighing  a  quarter  of  an  ounce,  and 
longer  than  the  diameter  of  the 
ring. 

117.  When  the  machine    is  to 
be  used,  the  weight,  A,  is  drawn  up 
to  the  top  of  the  scale,  and  the  ring 
and  stage  are  placed  a  certain  num- 
ber of  inches  from  each  other.    The 
small  bar,  M,  is  then  placed  across 
the  weight,  A,  by  means  of  which 
it  is  made  slowly  to  descend.  When 
it   has  descended  to  the  ring,  the 
small  weight,  M,  is  taken  off  by  the 
ring,  and  thus  the  two  weights  are 
left  equal  to  each  other.     Now  it 
must  be  observed,  that  the  motion 
and  descent  of  the  weight,  A,  are  en- 
tirely  owing  to  the  gravitating  force 

of  the  weight,  M,  until  it  arrives  at  the  ring,  R,  when  the  ac- 
tion of  gravity  is  suspended,  and  the  large  weight  continues  to 
move  downward  to  the  stage,  in  consequence  of  the  velocity  it 
had  acquired  previously  to  that  time. 

118.  To  comprehend  the  accuracy  of  this  machine,  it  must 
be  understood  that  the  velocities  of  gravitating  bodies  are  sup- 
posed to  be  equal,  whether  they  are  large  or  small,  this  being 
the  case  when  no  calculation  is  made  for  the  resistance  of  the 
air.     Consequently,  the  weight  of  a  quarter  of  an  ounce  placed 
on  the  large  weight,  A,  is  a  representative  of  all  other  solid 

116  Describe  the  operation  of  Mr.  Atwood's  machine  for  estimating  the  velocities 
of  falling  bodies.  117.  After  the  sm;Jl  weight  is  taken  off  hy  the  ring,  why  does  the 
large  weight  continue  to  descend  1  118.  Does  his  machine  show  the  actual  velocity 
of  a  falling  body,  or  only  its  inerease  1 


Atwoo&s  Machine. 


GRAVITY.  29 

descending  bodies.  The  slowness  of  its  descent,  \vhen  com- 
pared with  freely  gravitating  bodies,  is  only  a  convenience  by 
which  its  motion  can  be  accurately  measured,  for  it  is  the  in- 
crease of  velocity  which  the  machine  is  designed  to  ascertain, 
and  not  the  actual  velocity  of  falling  bodies. 

119.  Xow  it  will  be  readily  comprehended,  that  in  this  re- 
spect it  makes  no  difference  how  slowly  a  body  falls,  provided 
it  follows  the  same  laws  as  other  descending  bodies,  and  it  has 
already  been  stated,  that  all  estimates  on  this  subject  are  made 
from  the  known  distance  a  body  descends  during  the  first  sec- 
ond of  time. 

120.  It  follows,  therefore,  that  if  it  can  be  ascertained  ex- 
actly, how  much  faster  a  body  falls  during  the  third,  fourth,  or 
fifth  second,  than  it  did  during  the  first  second,  we  should  be 
able  to  estimate  the  distance  it  would  fall  during  all  succeeding 
seconds. 

121.  If,  then,  by  means  of  a  pendulum  beating  seconds,  the 
weight,  A,  should  be  found  to  descend  a  certain  number  of  inches 
during  the  first  second,  and  another  certain  number  during  the 
next  second,  and  so  on,  the  ratio  of  acceleration  would  be  pre- 
cisely ascertained,  and  could  be  easily  applied  to  the  falling  of 
other  bodies;  and  this  is  the  use  to" which  this  instrument  is 
applied. 

122.  It  will  be  readily  conceived,  that  solid  bodies  falling  from 
great  heights,  must  ultimately  acquire  an  amazing  velocity  by 
this  proportion  of  increase.     An  ounce  ball  of  lead,  let  fall  from 
a  certain  height  toward  the  earth,  would  thus  acquire  a  force 
ten  or  twenty  times  as  great  as  when  shot  out  of  a  rifle. 

123.  By  actual  calculation,  it  has  been  found  that  were  the 
moon  to  lose  her  projectile  force,  which  counterbalances  the 
earth's  attraction,  she  would  fall  to  the  earth  in  four  days  and 
twenty   hours,  a  distance  of   240,000  miles.     And  were  the 
earth's  projectile  force  destroyed,  it  would  fall  to  the  sun,  with- 
out resistance,  in  sixty-four  days  and  ten  hours,  a  distance  of 
95,000,000  of  miles. 

124.  Every  one  knows,  by  his  own  experience,  the  different 
effects  of  the  same  body  felling  from  a  great,  or  small  height. 
A  boy  will   toss  up  his  leaden  bullet    and    catch  it  with  his 
hand,  but  he  soon  learns,  by  its  painful  effects,  not  to  throw  it 

121.  By  what  means  is  the  ratio  of  descent  found  ?  122.  Would  it  be  possible  for  a 
rifl- -ball  to  acquire  a  greater  force  by  falling,  than  if  shot  from  a  rifle  ?  123.  How 
long  would  it  fate  the  moon  to  come  to  the  earth,  according  to  the  Jaw  of  increased 
velocity  ?  How  long  would  it  take  the  earth  to  fall  to  the  sun  ?  124.  What  familiar 
illustrations  are  given  of  the  force  acquired  by  the  velocity  of  falling  bodies  1 


30  GRAVITY. 

too  high.  The  effects  of  hailstones  on  window-glass.,  animals, 
and  vegetation,  are  often  surprising,  and  some  times  calamitous 
illustrations  of  the  velocity  of  falling  bodies. 

125.  It  has  been  already  stated  that  the  velocities  of  solid 
bodies,  falling  from  a  given  height  toward  the  earth,  are  equal, 
or  in  other  words,  that  an  ounce  ball  of  lead  will  descend  in  the 
same  time  as  a  pound  ball  of  lead. 

This  is  true  in  theory,  and  in  a  vacuum,  but  there  is  a  slight 
difference  in  this  respect  in  favor  of  the  velocity  of  the  larger 
body,  owing  to  the  resistance  of  the  atmosphere.  We,  how- 
ever, shall  at  present  consider  all  solids,  of  whatever  size,  as  de- 
scending  through  the  same  spaces  in  the  same  times,  this  being 
exactly  true  when  they  pass  without  resistance. 

126.  To  comprehend  the  reason  of  this,  we  have  only  to  con- 
sider, that  the  attraction  of  gravitation  in  acting  on  a  mass  of 
matter,  acts  on  every  particle  it  contains  ;  and  thus  every  parti- 
cle is  drawn  down  equally,  and  with  the  same  force.     The  ef- 
fect of  gravity,  therefore,  is  in  exact  proportion  to  the  quantity 
of  matter  the  mass  contains,  and  not  in  proportion  to  its  bulk. 

127.  A  ball  of  lead  of  a  foot  in  diameter,  and  one  of  wood 
of  the  same  diameter,  are  obviously  of  the  same  bulk ;  but  the 
lead  contains  twelve  particles  of  matter  where  the  wood  con- 
tains only  one,  and  consequently  will  be  attracted  with  twelve 
times  the  force,  and  therefore  will  weigh  twelve  times  as  much. 

128.  Attraction  proportionable  to  the  quantity  of  matter. — 
If,  then,  bodies  attract  each  other  in  proportion  to  the  quantities 
of  matter  they  contain,  it  follows  that  if  the  mass  of  the  earth 
were  doubled,  the  weights  of  all  bodies  on  its  surface  would  also 
be  doubled ;  and  if  its  quantity  of  matter  were  tripled,  all  bodies 
would  weigh  three  times  as  much  as  they  do  at  present. 

129.  It  follows,  also,  that  two  attracting  bodies,  when  free  to 
move,  must  approach  each  other  mutually.     If  the  two  bodies 
contain  .like  quantities  of  matter,  their  approach  will  be  equally 
rapid,  and  they  will  move  equal  distances  toward  each  other. 
But  if  the  one  be  small  and  the  other  large,  the  small  one  will 
approach  the  other  with  a  rapidity  proportioned  to  the  less 
quantity  of  matter  it  contains. 

130.  It  is  easy  to  conceive,  that  if  a  man  in  one  boat  pulls  at 

125  Will  a  small  and  a  large  body  fill  through  the  same  space  in  the  same  time  ? 
126  On  what  parts  of  a  mass  of  matter  does  the  force  of  cnivity  act  ?  Is  the  effect 
of  gravity  in  proportion  to  bulk,  or  qu-uitity  of  matter  ?  127.  What  is  the  difference 
between  a  ball  of  lead  and  one  of  wood,  of  the  same  size"?  128.  Were  the  mass  of 
the  earth  doubled,  how  much  more  should  we  weigh  1  129.  Suppose  one  body 
moving  toward  another,  three  times  as  larsre,  by  the  force  of  gravity  what  would  be 
their  proportional  velocities  1  130.  How  is  this  illustrated  J 


GRAVITY.  31 

a  rope  attacned  to  another  boat,  the  two  boats,  if  of  the  same 
size,  will  move  toward  each  other  at  the  same  rate ;  but  if  the 
one  be  large,  and  the  other  small,  the  rapidity  with  which  each 
moves  will  be  in  proportion  to  its  size,  the  large  one  moving 
with  as  much  less  velocity  as  its  size  is  greater. 

131.  A  man  in  a  boat,  pulling  a  rope  attached  to  a  ship, 
seems  only  to  move  the  boat ;  but  that  he  really  mo\  es  the 
ship  is  certain,  when  it  is  considered  that  a  thousand  boa  s  pull- 
ing in  the  same  manner  would  make  the  ship  meet  them  half 
way. 

It  appears,  therefore,  that  an  equal  force  acting  on  bodies 
containing  different  quantities  of  matter,  moves  them  with  dif- 
ferent velocities,  and  that  these  velocities  are  in  an  inverse  pro 
portion  to  their  quantities  of  matter. 

In  respect  to  equal  forces,  it  is  obvious  that  in  the  case  of 
the  ship  and  single  boat,  they  were  moved  toward  each  other 
by  the  same  force,  that  is,  the  force  of  a  man  pulling  by  a 
rope.  The  same  principle  holds  in  respect  to  attraction,  for  all 
bodies  attract  each  other  equally,  according  to  the  quantities  of 
matter  they  contain ;  and  since  all  attraction  is  mutual,  no  body 
attracts  another  with  a  greater  force  than  that  by  which  it  is  at- 
tracted. 

132.  Suppose  a  body  to  be  placed  at  a  distance  from  the 
earth,  weighing  two  hundred  pounds ;  the  earth  would  then 
attract  the  body  with  a  force  equal  to  two  hundred  pounds,  and 
the  body  would  attract  the  earth  with  an  equal  force,  other- 
wise their  attraction  would  not  be  equal  and  mutual.     Another 
body,  weighing  ten  pounds,  would  be  attracted  with  a  force 
equal  to  ten  pounds,  and  so  of  all  bodies  according  to  the  quan- 
tity of  matter  they  contain  ;  each  body  rJeing  attracted  by  the 
earth  with  a  force  equal  to  its  own  weight,  and  attracting  the 
earth  with  an  equal  force. 

133.  If,  for  example,  two  boats  be  connected  by  a  rope,  and 
a  man  in  one  of  them  pulls  with  a  force  equal  to  100  pounds, 
it  is  plain  that  the  force  on  each  vessel  would  be  100  pounds. 
For  if  the  rope  were  thrown  over  a  pulley,  and  a  man  were 
to  pull  at  one  end  with  a  force  of  100  pounds,  it  is  plain  it 
would  take  1 00  pounds  at  the  other  end  to  balance.    See  Fig.  11. 

131.  Does  a  large  body  attract  a  small  one  with  any  more  force  than  it  is  attracted  ? 
13:2.  Suppose  a  body  weighing  200  pounds  to  be  placed  at  a  distance  from  the  earth, 
with  how  much  force  does  the  earth  attract  the  body  ?  With  what  force  does  rhe 
body  aitract  the  earth  ?  133.  Suppose  a  man  in  one  boat  pulls  with  'a  force  of  100 
pounds  at  a  rope  fastened  to  another  boat,  what  would  be  the  force  on  each 
How  is  this  illustrated  1 


32  ASCENT    OF    BODIES. 

FIG.  11. 


Attraction  illustrated. 

134.  Attracting  bodies  approach  each  other. — It  is  inferred 
from  the  above  principles,  that  all  attracting  bodies  which  are 
free  to  move,  mutually  approach  each  other,  and  therefore  that 
the  earth  moves  toward  every  body  which  is  raised  from  its  sur- 
face, with  a  velocity  and  to  a  distance  proportional  to  the  quan- 
tity of  matter  thus  elevated  from  its  surface.     But  the  velocity 
of  the  earth  being  as  many  times  less  than  that  of  the  falling 
body  as  its  mass  is  greater,  it  follows  that  its  motion  is  not  per- 
ceptible to  us. 

The  following  calculation  will  show  what  an  immense  mass 
of  matter  it  would  take,  to  disturb  the  earth's  gravity  in  a  per- 
ceptible manner. 

135.  If  a  ball  of  earth,  equal  in  diameter  to  the  tenth  part 
of  a  mile,  were  placed  at  the  distance  of  the  tenth  part  of  a 
mile  from  the  earth's  surface,  the  attracting  -powers  of  the  two 
bodies  would  be  in  the  ratio  of  about  512  millions  of  millions 
to  one.     For  the  earth's  diameter  being  about  8000  miles,  the 
two  bodies  would  bear  to  each  other  about  this   proportion. 
Consequently,  if  the  tenth  part  of  a  mile  were  divided  into  512 
millions  of  millions  of  equal  parts,  one  of  these  parts  would  be 
nearly  the  space  through  which  the  earth  would  move  toward 
the  falling  body.     Now,  in  the  tenth  part  of  a  mile  there  are 
about  6400  inches,  consequently  this  number  must  be  divided 
into  512  millions  of  millions  of  parts,  which  would  give  the 
eighty  thousand  millionth  part  of  an  inch  through  which  the 
earth  would  move  to  meet  a  body  the  tenth  part  of  a  mile  in 
diameter. 

ASCENT    OF    BODIES. 

136.  Having  now  explained  and  illustrated  the  influence  of 
gravity  on  bodies  moving  downward  and  horizontally,  it  remains 
to  show  how  matter  is  influenced  by  the  same   power  when 
bodies  are  thrown  upward,  or  contrary  to  the  force  of  gravity. 

134.  Do  all  attracting  bodies  approach  each  other  1  Suppose  the  body  falls  toward 
the  earth,  is  the  earth  set  in  motion  by  its  attraction  ?  Why  is  not  the  earth's  motion 
toward  it  perceptible  ?  135.  What  d.stance  would  a  body,  the  tenth  part  of  a  mile  in 
diameter,  placed  at  the  distance  of  a  tenth  part  of  a  mile,  attract  the  earth  toward  it? 


FALLING    BODIES.  33 

137.  What  has  been  stated  in  respect  to  the  ve-      FIG  12-  ^ 
locity  of   falling  bodies  is   reversed  in  respect  to        d  [" 
those  which  are  thrown  upward,  for  as  the  motion 

of  a  falling  body  is  increased  by  the  action  of  gravi- 
ty, so  it  is  retarded  by  the  same  force  when  pro- 
jected from  the  center  of  gravity. 

A  bullet  shot  upward,  every  instant  loses  a  part 
of  its  velocity,  until  having  arrived  at  the  highest 
point  from  whence  it  was  thrown,  it  then  returns 
again  to  the  earth. 

138.  The  same  law  that  governs  a  descending 
body,  governs  an  ascending  one,  only  that  their  mo- 
tions are  reversed. 

139.  The  same  ratio  is  observed  to  whatever  dis- 
tance the  ball  is  propelle'd,  for  as  the  height  to 
which  it  is  thrown  may  be  estimated  from  the  space, 
it  passes  through  during  the  first  second,  so  its  re- 
turning velocity  is  in  a  like  ratio  to  the  height  to 
which  it  was  sent. 

140.  This  will  be  understood  by  Fig.  12.     Sup- 
pose a  ball  to  be  propelled  from  the  point  a,  with 
a  force  which  would  carry  it  to  the  point  b  in  the 
first  second,  to  c  in  the  next,  and  to  d  in  the  third 
second.     It  would  then  remain  nearly  stationary  for 
an  instant,  and  in  returning  would  pass  through  the 
same  spaces  in  the  same  time,  only  that  its  direc- 
tion would  be  reversed.     Thus,  it  will  fall  from  d 
to  c  in  the  first  second,  to  b  in  the  next,  and  to  a  in 
the  third. 

141.  Now  the  momentum  of  a  moving  body  is  as 
its  velocity  and  its  quantity  of  matter,  and  hence 

the  same  ball  will  fall  with  the  same  force  that  it  rises.  For  in- 
stance, a  ball  shot  out  of  a  rifle,  with  force  sufficient  to  overcome 
a  certain  impediment,  on  returning  would  again  overcome  the 
same  impediment. 

142.  It  has  been  doubted,  even  by  good  authority,  whether 
the  principle  above  enunciated  is  true — that  is,  whether  a  rising 
and  a  falling  body  observe  the  same  law  of  motion,  only,  that 
they  are  reversed.     On  this  point  we  quote  Dr.  Lardner,  who, 
perhaps,  is  not  inferior  to  any  other  authority. 

137.  What  effVct  does  the  force  of  gravity  have  on  bodies  moving  upward  ?  138.  Are 
upward  and  downward  motion  governed  by  the  same  laws  1  140.  Explain  Fig.  12. 
What  is  the  difference  between  the  upward  and  returning  velocity  of  the  same  body? 
HI.  What  is  said  of  (he  returning  force  ol  a  rifle-ball  1  142.  What  doubts  have  been 
expressed  on  this  subject  ?  ^ 


34  MOTION    ON    INCLINED    PLANES. 

'•  143.  All  the  circumstances  attending  the  accelerated  de- 
scent of  falling  bodies,  are  exhibited  in  a  reversed  order  when  a 
body  is  projected  upward. 

"  Thus,  if  a  body  be  projected  vertically  upward,  with  the  ve- 
locity which  it  would  acquire  in  falling  freely  during  one  second, 
the  body  so  projected  will  rise  exactly  to  the  height  from  which 
it  would  have  fallen  in  one  second,  and  at  that  point  of  its  as- 
cent, it  will  have  the  velocity  which  it  would  have  at  the  same 
point,  if  it  had  descended." — Hand  Book  of  Natural  Philoso- 
phy, (London,  1851,)  p.  116. 

144.  It  has  been  estimated  that  a  leaden  ball  (122)  falling 
from  a  sufficient  height,  would  acquire  a  much  greater  force 
than  if  shot  from  a  rifle. 

It  is  understood  that  these  estimates  refer  only  to  dense  bullets, 
as  those  of  lead,  or  other  metals,  on  which  the  atmosphere  has 
the  least  resistance. 

145.  It  is  stated  that  attempts  have  been  made  to  test  this 
principle  by  shooting  rifle-balls  vertically,  and  observing  with 
what  force  they  descended,  by  the  depth  they  penetrated  wood- 

,en  impediments. 

But  this  would  hardly  be  within  the  art  of  gunnery,  unless 
the  mark  erected  for  the  returning  ball  should  be  more  exten- 
sive than  experimenters  would  be  willing  to  construct. 

MOTION    ON    INCLINED    PLANES. 

146.  Bodies  falling  down  inclined  planes  follow  the  same 
laws  of  motion  as  those  falling  freely,  only  that  their  velocities 
are  diminished  in  proportion  as  the  planes  are  more  or  less  in- 
clined. 

14Y.  This  is  illustrated  by  Fig.  13,  where  let  b  be  an  inclined 
plane,  and  A,  G,  the  vertical  line  of  the  same  length,  the  letters 
on  each  marking  the  points  to  which  the  falling  body  is  sup- 
posed to  reach  in  1,  2,  3,  4,  and  5  seconds.  Now  suppose  two 
balls  to  be  dismissed  at  the  same  instant  from  A,  the  one  fall- 
ing freely,  and  the  other  along  the  plane.  Then,  to  find  the 
difference  in  their  velocities,  draw  perpendicular  lines  from  the 
points,  1,  2,  3, 4,  and  5,  along  the  inclined  plane,  and  extend  these 
lines  to  B,  C,  D,  E,  F,  G,  of  the  vertical  line,  and  these  points 
will  respectively  mark  the  difference  in  their  velocities.  Thus,  at 
the  end  of  the  first  second,  one  of  the  balls  will  arrive  at  B,  and 

143.  What  is  the  quotation  from  Dr.  Lardner?  144.  What  estimates  have  been 
made  with  respect  to  the  fall  of  a  rifle-ball  ?  145.  What  is  said  of  the  experiment  of 
shooting  rifle-balls  vertically  1  146.  What  are  the  laws  of  motion  down  inclined 
planes  ?  147.  Explain  Fig.  13. 


FALL    OF    LIGHT    BODIES. 


35 


Fall  on  Inclined  Plane. 


the  other  at  b,  and  so  in  these  propor-  FIG<  13* 

tions  until  they  fall  to  the  earth. 

148.  It  will   therefore  be  observed, 
that  although  the  ball  which  falls  down 
the  plane  is  retarded  in  its  motion  by 
friction,  still  it  follows  the  same  law  as  the 
other,  both  being  uniformly  accelerated 
in  their  descent  by  the  force  of  gravity. 

FALL    OF    LIGHT    BODIES. 

149.  Jt  has   been   stated    that    the 
earth's  attraction  acts  equally  on  all 
bodies  containing  equal  quantities   of 
matter,  and  that  in  v'acuo,  all  bodies, 
whether  large  or  small,  descend  from  the 
same  heights  in  the  same  time.   (125.) 

There  is,  however,  a  great  differ- 
ence in  the  quantities  of  matter  which 
bodies  of  the  same  birlk  contain,  and 
consequently  a  difference  in  the  resist- 
ance which  they  meet  with  in  passing 
through  the  air. 

150.  Now,  the  fall  of  a  body  containing  a  large  quantity  of 
matter  in  a  small  bulk,  meets  with  little  comparative  resistance, 
while  the  fall  of  another,  containing  the  same  quantity  of  mat- 
ter, but  of  larger  size,  meets  with  more  in  comparison,  for  two 
bodies  of  the  same  size,  meet  with  exactly  the  same  resistance. 
Thus,  if  we  let  fall  a  ball  of  lead,  and  another  of  cork,  of  two 
inches  in  diameter  each,  the  lead  will  reach  the  ground  before 
the  cork,  because,  though  meeting  with  the  same  resistance,  the 
lead  has  the  greatest  power  of  overcoming  it. 

151.  This,  however,  does  not  affect  the  truth  of  the  general 
law,  already  established,  that  the  weights  of  bodies  are  as  the 
quantities  of  matter  they  contain.     It  only  shows  that  the  pres- 
sure of  the  atmosphere  prevents  bulky  and  porous  substances 
from  falling  with  the  same  velocity  as  those  which  are  compact 
or  dense. 

152.  Were  the  atmosphere  removed,  all  bodies,  whether  light 
or  heavy,  large  or  small,  would  descend  with  the  same  velocity. 
This  has  besn  ascertained  by  experiment  in  the  following  manner : 

143.  What  does  the  explanation  of  the  fiirure  prove  ?  149.  What  is  said  of  the  fall 
of  bodies  ?  150  Why  will  not  a  sack  of  feathers  and  a  stone  of  the  same  size  fall 
through  the  air  in  the  same  time  ?  151.  Does  this  afftct  the  truth  of  the  general  law, 
that  the  weights  of  bodies  are  as  their  quantities  of  matter!  152.  What  would  be  the 
effect  on  the  fall  of  light  and  heavy  bodies,  were  the  atmosphere  removed  7 


36 


MOTION. 


The  air-pump  is  an  instrument '  by 
means  of  which  the  air  can  be  pumped 
out  of  a  close  vessel,  as  will  be  seen  under 
the  article  Pneumatics.  Taking  this  for 
granted  at  present,  the  experiment  is  made 
in  the  following  manner : 

153.  On  the  plate  of  the  air-pump,  A, 
place  the  tall  jar,  B,  which  is  open  at  the 
bottom,  and  has  a  brass  cover  fitted  close- 
ly to  the  top.  Through  the  cover  let  a 
wire  pass,  air-tight,  having  a  small  cross 
at  the  lower  end.  On  each  side  of  this 
cross  place  a  little  stage,  and  so  contrive 
them  that  by  turning  the  wire  by  the 
handle,  C,  these  stages  shall  be  upset.  On 
one  of  the  stages  place  a  guinea  or  piece 
of  lead,  and  on  the  other  place  a  feather. 
When  this  is  arranged,  let  the  air  be  ex- 
hausted from  the  jar  by  the  pump,  and 
then  turn  the  handle,  C,  so  that  the  guinea 
and  feather  may  fall  from  their  places,  and 
it  will  be  found  that  they  will  both  strike 
the  plate  at  the  same  instant.  Thus  is  it 
demonstrated,  that  were  it  not  for  the  re- 
sistance of  the  atmosphere,  a  bag  of  feathers  and  one  of 
guineas  would  fall  from  a  given  height  with  the  same  velocity 
and  in  the  same  time. 


CHAPTER  III. 

MOTION. 

154.  MOTION  may  be  defined,  a  continued  change  of  place, 
with  regard  to  a  fixed  point. 

155.  Without  motion  there  would  be  no  rising  »or  setting 
of  the  sun — no  change  of  seasons — no  fall  of  rain — no  building 
of  houses,  and  finally  no  animal  life.     Nothing  can  be  done 
without  motion,  and  therefore  without  it,  the  whole  universe 
would  be  at  rest  and  dead. 


153.  How  is  it  proved  that  a  feather  and  a  guinea  will  fall  through  equal  spaces  in 
the  same  time,  where  there  is  no  resistance?    154.  How  will  you  define 
165.  What  would  be  the  consequence  were  all  motion  to  cease  ? 


VELOCITY    OF    MOTION.  37 

156.  In  the  language  of  philosophy,  the  power  which  puts 
%  body  in-  motion  is  called  force.  Thus,  it  is  the  force  of  gravi- 
ty that  overcomes  the  inertia  of  bodies,  and  draws  them  toward 
the  earth.  The  force  of  water  and  steam  gives  motion  to  ma- 
chinery, &c. 

157*  For  the  sake  of  convenience,  and  accuracy  in  the  use  of 
terms,  motion  is  divided  into  two  kinds,  viz.  absolute  and  relative. 

158.  Absolute  motion  is  a  change  of  place  with  regard  to  a 
fixed  point,  and  is  estimated  without  reference  to  the  motion  of 
any  other  body.     When  a  man  rides  along  the  street,  or  when 
a  vessel  sails  through  the  water,  they  are  both  in  absolute  motion. 

159.  Relative  motion  is  a  change  of  place  in  a  body,  with 
respect  to  another  body,  also  in  motion,  and  is  estimated  from 
that  other  body  exactly  as  absolute  motion  is  from  a  fixed  point. 

160.  The  absolute  velocity  of  the  earth  in  its  orbit  from  west 
to  east,  is  68,000  miles  in  an  hour ;  that  of  Mars,  in  the  same 
direction,  is  55,000  miles  per  hour.     The  earth's  relative  ve- 
locity, in  this  case,  is  13,000  miles  per  hour  from  west  to  east. 
That  of  Mars,  comparatively,  is  13,000  miles  from  east  to  west, 
because  the  earth  leaves  Mars  that  distance  behind  her,  as  she 
would  leave  a  fixed  point. 

161.  Best,  in  the  common  meaning  of  the  term,  is  the  op- 
posite of  motion,  but  it  is  obvious  that  rest  is  often  a  relative 
term,  since  an  object  may  be  perfectly  at  rest  with  respect  to 
some  things,  and  in  rapid  motion  in  respect  to  others. 

162.  Thus,  a  man  sitting  on  the  deck  of  a  steamboat,  may 
move  at  the  rate  of  fifteen  miles  per  hour,  with  respect  to  the 
land,  and  still  be  at  rest  with  respect  to  the  boat.     And  so,  if 
another  man  was  running  on  the  deck  of  the  same  boat  at  the 
rate  of  fifteen  miles  the  hour  in  a  contrary  direction,  he  would 
be  stationary  in  respect  to  a  fixed  point,  and  still  be  running 
with  all  his  might,  with  respect  to  the  boat. 

VELOCITY    OF    MOTION. 

163.  Velocity  is  the  rate  of  motion  at  which  a  body  moves 
from  one  place  to  another. 

Velocity  is  independent  of  the  weight  or  magnitude  of  the 
moving  body.  Thus,  a  cannon-ball  and  a  musket-ball,  both 
living  at  the  rate  of  a  thousand  feet  in  a  second,  have  the  same 
velocities. 

156  What  is  that  power  called  which  puts  a  body  in  motion  7  157.  How  is  motion 
divided  1  158.  What  is  absolute  motion  1  159.  What  is  relative  motion  ?  160.  What 
is  the  earth's  relative  velocity  in  respect  to  Mars  7  161.  What  is  rest  1  162.  In  what 
respect  is  a  man  in  a  stea'mboat  at  rest,  and  in  what  respect  does  he  move11 
163.  What  is  velocity  7 


38 


VELOCITY    OF    MOTION. 


164.  Velocity  is  said  to  be  uniform,  when  the  moving  body 
passes  over  equal  spaces  in  equal  times.     If  a  steamboat  moves 
at  the  rate  of  ten  miles  every  hour,  her  velocity  is  uniform.     The 
revolution  of  the  earth  from  west  to  east  is  a  perpetual  exam- 
ple of  uniform  motion. 

165.  Velocity  is  accelerated,  when  the  rate  of  motion  is  in- 
creased, and  the  moving  body  passes  through  unequal  spaces  in 
equal  times.     Thus,  when  a  falling  body  moves  sixteen  feet 
during  the  first  second,  and  forty-eight  feet  during  the  next 
second,  and  so  on,  its  velocity  is  accelerated.     A  body  falling 
from  a  height  freely  through  the  air,  is  the  most  perfect  exam- 
ple of  this  kind  of  velocity. 

166.  Retarded  velocity,  is  when  the  rate   of  motion  of  the 
body  is  constantly  decreased,  and  it  is  made  to  move  slower 
and  slower.  .  A  ball  thrown  upward  into  the  air,  has  its  veloci- 
ty constantly  retarded  by  the  attraction  of  gravitation,  and  con- 
sequently, it  moves  slower  every  moment.   (137.) 

VELOCITIES    OF    CERTAIN    MOVING    BODIES. 


167.  Objects  moving  :  — 
Man  walking,           

Miles  pe 
hour. 
.     .              3 

r 

Feet  per 
second. 
44 

Horse  trotting,         
Swiftest  race-horse,      ..... 
Railway  train,  (Eno-lish) 

.     .            7 
.     .          60 
.     .          32 

Itt 

88 
47 

.    .          18 

26 

"           (Belgian)         .     .     . 

25 

36 

"           (French)          .     .     . 

.     .          27 

40 

"           (German)        .     .     . 

24 

35 

Swift  "EnoTish  steamers 

14 

20 

American  steamers  on  the  Hudson, 
Fast  sailing  vessels, 

.     .           18 
10 

26 
14 

Current  of  slow  rivers 

3 

41 

"       of  rapid  rivers, 

.     .            7 

10 

Moderate  wind,                   ••  ''-  -"."  »:  "„ 

7 

10 

A  storm,  with  wind, 

36 

52 

A  hurricane,  in  hot  climates,      .     . 
Air  rushing  into  a  vacuum,         .     . 
Common  musket-ball,                  .     . 
A  rifle  -ball,                              .     .     . 

.     .          80 
.     .        884 
.     .        850 
.     .       1000 

117 

1296 
1246 
1466 

A  24-lb.  cannon-ball,             .     .     . 

.     .       1600 

2346 

.     .         466 

683 

Sound,  heat  at  32°,                .     .     . 

748 

1090 

"         do  at  60° 

762 

1118 

Earth's  velocity  round  the  sun, 
"         diurnal  motion  at  equator, 

.     .   67,374 
.     .       1037 

98,815 
1520 

164.  When  is  velocity  uniform  ?  165.  When  is  velocity  accelerated  ?  Give  illus- 
trations of  these  two  kinds  of  velocity.  166.  What  is  meant  by  retarded  velocity  1 
Give  an  example  of  retarded  velocity 


MOMENTUM.  39 

168.  The  above,  from  Lardner's  Mechanics,  may  be  useful  for 
occasional  reference.     We  have  omitted  the  fractional  parts  with 
respect  to  the  seconds,   as  being  difficult  to  remember,   and 
useless  for  the  present  purpose.     In  regard  to  American  loco- 
motive speed,  it  is  at  the  present  time  probably  nearly  one-third 
too  small.     The  comparative  velocities  of  balls  from  fire-arms 
differ  from  those  given  by  some  other  authorities,  but  on  this 
subject  we  have  made  no  experiments. 

FORCE,    OR    MOMENTUM    OF    MOVING    BODIES. 

169.  The  velocities  of  bodies  are  equal,  when   they  pass  over 
equal  spaces  in  the  same  times  ;   but  the  force  with  which  bodies, 
moving  at  the  same  rate,  overcome  impediments,  is  in  propor- 
tion  to  the  quantity  of  matter   they  contain.     This  power,  or 
force,  is  called  the  momentum  of  the  moving  body. 

170.  Thus,  if  two  bodies  of  the  same  weight  move  with  the 
same  velocity,  their  momenta  will  be  equal. 

171.  Two  vessels,  each  of  a  hundred  tons,  sailing  at  the  rate 
of  six  miles  an  hour,  would  overcome  the  same  impediments  or 
be  stopped  by  the  same  obstructions.     Their  momenta  would 
therefore  be  the  same. 

The  force  or  momentum  of  a  moving  body,  is  in  proportion 
to  its  quantity  of  matter,  and  its -velocity. 

172.  A  large  body  moving  slowly,  may  have  less  momenta 
than  a  small  one  moving  rapidly.     Thus,  a  bullet  shot  out  of  a 
gun,  moves  with  much  greater  force  than  a  stone  thrown  by  the 
hand. 

173.  The  momentum  of  a  body  is  found  by  multiplying  its 
quantity  of  matter  by  its  velocity  per  second.     Thus,  if  the 
velocity  be  2,  and  the  weight  2,  the  momentum  will  be  4.     If 
the  velocity  be  6,  and  the  weight  of  the  body  4,  the  momentum 
will  be  24. 

174.  If  a  moving  body  strikes  an  impediment,  the  force  with 
which  it  strikes,  and  the  resistance  of  the  impediment,  are  equal. 
Thus,  if  a  boy  throw  his  ball  against  the  side  of  the  house,  with 
the  force  of  3,  the  house  resists  it  with  an  equal  force,  and  the 
ball  rebounds.     If  he  throws  it  against  a  pane  of  glass  with  the 
same  force,  the  glass  having  only  the  power  of  2  to  resist,  the  ball 
will  go  through  the  glass,  still  retaining  one-third  of  its  force. 

168.  What  is  said  of  the  speed  of  our  locomotives  1  169.  What  is  meant  by  ths 
momentum  of  a  body  ?  170.  When  will  the  momentum  of  two  bodies  be  equal  ? 
171  Gire  an  example.  172.  When  has  a  small  body  a  greater  momentum  than  a 
lanreone?  173  By  what  rule  is  the  momentum  of  a  body  found  ?  174.  When  a 
moving  body  strikes  au  impediment,  which  receives  the  greatest  shock  ? 


40 


MOMENTUM. 


FIG.  15. 


175.  PILE  DRIVER. — This  machine  consists  of  a  frame  and 
pulley,  by  which  a  large  piece  of  cast  iron,  called  the  hammer, 
is  raised  to  the  height  of  30  or  40  feet,  and  then  let  fall  on  the 
end  of  a  beam  of  wood  called  a  pile,  and  by  which  it  is  driven 
into  the  ground.     When  the  hammer  is  large,  and  the  height 
considerable,  the  force,  or  momentum,  is  tremendous,  and  unless 
ths  pile  is  hooped  with  iron,  will  split  it  into  fragments. 

176.  Now  the  momentum  of  a  body  being  in  proportion  to 
its  weight  and  velocity  conjointly,  to  find  it,  we  must  multiply 
their  two  sums  together. 

Suppose  then  the  hammer,  weighing  2000  pounds,  is  ele- 
vated two  seconds  of  time  abovje  the  head  of  the  pile,  then, 
according  to  the  law  of  falling  bodies,  (110,)  it  would  fall  64  feet, 
this  being  the  rate  of  its  velocity.  Then  64  X  2000,  being  the 
velocity  and  quantity  of  matter,  gives  64  tons  as  the  momen- 
tum. But  according  to  the  same  law,  this  force  is  immensely 
increased  by  a  small  increase  of  time,  for  if  we  add  two  seconds 
of  time,  the  rate  of  velocity  at  the  instant  of  striking  would  be 
256  feet  per  second,  and  thus  256  X  2000  =  512,000  pounds,  or 
256  Ions. 

177.  ACTION  AND  REACTION 
EQUAL.  —  From     observations 
made  on  the  effects  of  bodies 
striking  each  other,  it  is  found 
that  action  and  reaction   are 
equal  •  or,  in  other  words,  that 
force  and  resistance  are  equal. 
Thus,    when  a    moving    body 
strikes  one  that  is  at  rest,  the 
body  at  rest  returns   the  blow 
with  equal  force. 

This  is  illustrated  by  the 
well-known  fact,  that  if  two 
persons  strike  their  heads  to- 
gether, one  being  in  motion, 
and  the  other  at  rest,  they  are 
both  equally  hurt. 

178.  The  philosophy  of  ac- 
tion and  reaction  is  finely  illustrated   by  a  number  of  ivory 
balls,  suspended  by  threads,  as  in  Fig.  15,  so  as  to  touch  each 

175.  What  is  a  pile  driver  ?  176.  If  the  hammer  of  this  machine  weighs  2000 
pounds,  and  falls  2  seconds,  what  will  be  the  momentum  ?  If  the  fall  be  3  seconds, 
what  is  the  momentum  1  177.  What  is  the  law  of  action  and  reaction  ?  How  is  this 
illustrated  7 


e  d  C  b 

Action  and  Reaction. 


REFLECTED    MOTION.  41 

other.  If  the  ball  a  be  drawn  from  the^erpendicular,  and  then 
let  fall,  so  a?  to  strike  the  one  next  to  it,  the  motion  of  the  falling 
ball  will  be  communicated  through  the  whole  series,  from  one  to 
the  other.  None  of  the  balls  except  /,  will,  however,  appear  to 
move.  This  will-  be  understood,  when  we  consider  that  the  reac- 
tion of  b  is  just  equal  to  the  action  of  a,  and  that  each  of  -the 
other  balls,  in  like  manner,  act,  and  react,  on.  the  other,  until 
the  motion  of  a  arrives  at  /,  which,  having  no  impediment,  or 
nothing  to  act  upon,  is  itself  put  in  motion.  It  is  therefore, 
reaction,  which  causes  all  the  balls,  except/,  to  remain  at  rest. 

180.  It  is  by  a  modification  of  the  same  principle,  that  rock- 
ets are  impelled  through  the  air.     The  stream  of  expanded  air, 
or  the  fire,  which  is  emitted  from  the  lower  end  of  the  rocket, 
not  only  pushes  against  the  rocket  itself,  but  against  the  atmos- 
pheric air,  which,  reacting  against  the  air  so  expanded,  sends 
the  rocket  along. 

181.  It  was  on  account  of  not  understanding  the  principles 
of  action  and  reaction,  that  the  man  undertook  to  make  a  fair 
wind  for  his  pleasure-boat,  to  be  used  whenever  he  wished   to 
sail.     He  fixed  an  immense  bellows  in  the  stern  of  his  boat,  not 
doubting  that  the  wind  from  it  would  carry  him  along.     But 
on  making,  the  experiment,  he  found  that  his  boat  went  back- 
ward instead  of  forward.     The  reason  is  plain.     The  reaction 
of  the   atmosphere  on  the  stream  of  wind    from  the  bellows, 
before  it  reached  the  sail,  moved  the  boat  in  a  contrary  direction. 

182.  Had  the  sail  received  the  whole  force  of  the  wind  from 
the  bellows,  the  boat  would  not  have  moved  at  all,  for  then, 
action  and  reaction  would  have  been  exactly  equal,  and  it  would 
have  been  like  a  man's  attempting  to  raise  himself  over  a  fence 
by  the  straps  of  his  boots. 

REFLECTED    3IOTIOX. 

183.  It  has  been  stated  (27)  that  all  bodies  when  once  set  in 
motion,  would  continue  to  move  straight  forward,  until  some 
impediment,  acting  in  a  contrary  direction,  should  bring  them 
to  rest  ;  continued  motion  without  impediment  being  a  conse- 
quence of  the  inertia  of  matter. 

184.  'Such  bodies  are  supposed  to  be  acted  upon  by  a  single 
force,  and  that  in  the  direction  of  the  line  in  which  they  move. 

179  When  one  of  the  ivory  balls  strikes  the  other,  why  does  the  most  distant  one  only 
move?  1^  On  what  principle  are  rockets  impelled  through  the  air?  181.  In  the 
experiment  With  the  boat  and  bellows,  why  did  the  boat  move  backward?  182. 
Why  would  it  not  have  moved  at  all  had  the  sail  received  all  the  wind  from  the  bel- 
lows? 183.  What  is  said  of  the  continuity  of  motion? 


42  REFLECTED    MOTION. 

TLi;s,  a  ball  sent  out  of  a  gun,  or  struck  by  a  bat,  turns  neither 
to  the  right  nor  left,  but  makes  a  curve  toward  the  earth,  in 
consequence  of  another  force,  which  is  the  attraction  of  gravita- 
tion, -and  by  which,  together  with  the  resistance  of  the  atmos- 
phere, it  is  finally  brought  to  the  ground. 

The  kind  of  motion  now  to  be  considered,  is  that  which  is 
produced  when  bodies  are  turned  out  of  a  straight  line  by  some 
force,  independent  of  gravity. 

A  single  force,  or  impulse,  sends  the  body  directly  forward, 
but  another  force,  not  exactly  coinciding  with  this,  will  give  it 
a  new  direction,  and  bend  it  out  of  its  former  course. 

1-85.  If,  for  instance,  two  moving  bodies  strike  each  other 
obliquely,  they  will  both  be  thrown  out  of  the  line  of  their  for- 
mer direction.  This  is  called  reflected  motion,  because  it 
observes  the  same  laws  as  reflected  light. 

186.  The  bounding  of  a  ball;  the  skipping  of  a  stone  over 
the  smooth  surface  of  a  pond ;  and  the  oblique  direction  of  an 
apple,  when  it  touches  a  limb  in  its  fall,  are  examples  of  reflected 
motion. 

By  experiments  on  this  kind  of  motion,  it  is  found  that  mov- 
ing bodies  observe  certain  laws,  in  respect  to  the  direction  they 
take  in  rebounding  from  any  impediment  they  happen  to  strike. 

187.  Thus,   a  ball,  striking  on  the  floor,  or  wall  of  a  room, 
makes  the  same  angle  in  leaving  the  point  where  it  strikes,  that 
it  does  in  approaching  it. 

188.  Suppose  a,  6, 
Fig.  16,  to  be  a  mar- 
ble floor,  and  c,  to  be 
an  ivory  ball,  which 
has  been  thrown  to- 
ward the  floor  in  the 

direction  of  the  line  Reflected  Motion. 

c  e ;  it  will  rebound 

in  the  direction  of  the  line  e  d,  thus  making  the  twTo  angles  f 

and  g  exactly  equal. 

189.  If  the  ball   approaches  the  floor  under  a  larger   or 
smaller  angle,  its  rebound  will  observe  the  same  rule.     Thus, 
if  it  fell  in  the  line  h  k,  Fig.  17,  its  rebound  would  be  in  the  line 


184.  Suppose  a  body  is  a-'ted  on.  and  set  in  motion  by  a  single  force,  in  what  direc- 
tion will  it  move?  185.  What  is  the  motion  called,  when  a  body  is  turned  out  of  a 
straight  line  by  another  force?  186.  What  illustrations  can  you  pive  of  reflected 
motion?  What  laws  are  observed  in  reflected  motion?  187.  Suppose  a  ball  to  be 
thrown  on  the  floor  in  a  certain  direction,  what  rule  will  it  observe  in  rebounding? 
188.  Explain  Fig  16. 


COMPOUND*  MOTION. 


43 


k  i,  and  if  it  was  drop-  FIG.  17. 

ped      perpendicularly 

from  I  to  k,  it  would 

return  in  the  same  line 

to  I.    The  angle  which 

the   ball    makes  with 

the  perpendicular  line, 

I  k,  in  its  approach  to 

the  floor,  is  called  the 

angle  of  incidence,  and 

that  which  it   makes 

in  departing  from  the 

floor  with  the  same  line,  is  called  the  angle  of  reflection,  and 

these  angles  are  always  equal. 


/c 

Equal  Angles. 


FIG.  18. 


COMPOUND    MOTION. 

190.  Compound  motion  is  (that  which  is  produced  by  two  or 
more  forces,  acting  in  different  directions,  on  the  same  body,  at 
the  same  time.     This  will  be  readily  understood  by  a  diagram. 

191.  Suppose  the  ball 
a,  Fig.  18,  to  be  moving 
with  a  certain  velocity  in 
the  line  6  c,  and  suppose 
that  at  the  instant  when  it 
came   to    the    point  a,  it 
should  be  struck  with  an 
equal  force  in  the  direction 
of  d  e,  then,  as  it  can  not 
obey  the  direction  of  both 
these  forces,  it  wTill  take  a 
course  between  them,  and 
fly  off  in  the  direction  of/. 

The    reason    of     this 

is   plain.     The  first  force  Compound  Motion. 

would  carry  the  ball  from 

b  to  c  ;  the  second  would  carry  it  from  d  to  e  ;  and  these  two 
forces  being  equal,  gives  it  a  direction  just  half  way  between  the 
two,  and  therefore  it  is  sent  toward  /. 

The  line    a  f,    is  called  the    diagonal   of  the  square,    and 

189.  What  is  the  angle  called,  which  the  ball  makes  in  approaching  the  floor  1 
What  is  !he  angle  called,  which  it  makes  in  leaving  the  floor  ?  What  is  the  difference 
between  these  angles  7  190.  What  is  compound  motion  ?  191.  Suppose  a  ball,  mov 
ing  with  a  certain" force,  to  be  struck  crosswise  with  the  same  force,  in  what  direc- 
tion will  it  move? 


44 


COMPOUND    MOTION. 


Diagonal  Motion. 


results  from  the  cross  forces,  b  and  d,  being  equal  to  each  other. 
If  one  of  the  moving  forces  is  greater  than  the  other,  then  the 
diagonal  lijie  will  be  lengthened  in  the  direction  of  the  greater 
force,  and  instead  of  being  the  diagonal  of  a  square,  it  will 
become  that  of  a  parallelogram. 

192.  Suppose  the  force 

in  the  direction  of  a  6,  ^G-  19« 

should  drive  the  ball  with 
twice  the  velocity  of  the 
cross  force  c  d,  Fig.  19, 
then  the  ball  would  go 
twice  as  far  from  the  line 
c  d,  as  from  the  line  b  a, 
and  ef  would  be  the  diag- 
onal of  a  parallelogram 
whose  length  is  double  its 
breadth. 

193.  Suppose  a  boat,  in 

crossing  a  river,  is  rowed  forward  at  the  rate  of  four  miles  an 
hour,  and  the  current  of  the  river  is  at  the  same  rate,  then  the 
two  cross  forces  will  be  equal,  and  the  line  of  the  boat  will  be 
the  diagonal  of  a  square,  as  in  Fig.  18.  But  if  the  current  be 
four  miles  an  hour,  and  the  progress  of  the  boat  forward  only 
two  miles  an  hour,  then  the  boat  will  go  down  stream  twice  as 
fast  as  she  goes  across  the  river,  and  her  path  will  be  the  diago- 
nal Of  a  parallelogram,  as  in  Fig.  19,  and  therefore,  to  make  the 
boat  pass  directly  across  the  stream,  it  must  be  rowed  toward 
some  point  higher  up  the  river  than  the  landing-place ;  a  fact 
well  known  to  boatmen. 

194.  CIRCUS  RIDER. — Those  who  have  seen  feats  of  horse- 
manship at  the  circus,  are  often  surprised  that  when  the  man 
leaps  directly  upward,  the  horse  does  not  pass  from  under  him, 
and  that  in  descending  he  does  not  fall   behind  the  animal. 
But  it  should  be  considered  that,  on  leaving  the  saddle,  the 
body  of  the  rider  has  the  same  velocity  as  that  of  the  horse ; 
nor  does  his  leaving  the  horse  by  jumping  upward,   in    nny 
degree  diminish  his  velocity  in  the  same  direction ;  his  motion 
being  continued  by  the  impulse  he  had  gained  from  the  animal. 
In  this  case,  the  body  of  the  man  describes  the  diagonal  of  a 
parallelogram,  one  side  of  which  is  in  the  direction  of  the  horse's 

192.  Suppose  it  to  be  struck  with  twice  its  former  force,  in  what  direction  will  it 
move?  What  is  the  line  af  Fig.  18,  called  ?  What  is  the  line  e  f.  Fig.  19,  called. 
How  are  these  figures  illustrated?  Explain  Figs.  18  and  19.  193.  Explain  the  motion  of 
the  boat  ?  Why  does  the  leaping  circus  rider  form  the  diagonal  of  a  parallelogram  ? 


CIRCULAR    MOTION. 


FIG.  20. 


motion,  and  the  other  perpendicularly  upward,  in  the  direction 
in  which  he  makes  the  leap. 

195.  This  will  be 
better  understood 
by  Fig.  20,  where 
the  two  forces  are 
illustrated.  Had 
the  rider  remained 
on  the  horse,  he 
would  have  reached 
that  point,  where  he 
meets  him  after  the 
leap  over  the  iron 

bar,  under  which  the  animal  passes.  This  force  the  rider  gains 
from  the  horse.  The  diagonal  force  is  the  result  of  his  own 
muscular  exertion,  and  by  which  he  raises  himself  above  the 
bar,  still  retaining  in  his  leap  the  velocity  of  the  horse,  and  thus 
regains  the  saddle,  as  though  he  had  not  left  it. 

The  motion  of  the  rider  is  through  the  section  of  a  sphere,  as 
shown  by  the  figure,  where  the  horse  and  rider  are  shown  before 
and  after  the  leap. 


Circus  Rider. 


CIRCULAR    MOTION. 

196.  Circular  motion  is  that  of  a  body  in  a  ring,  or  circle, 
and  is  produced  by  the  action  of  two  forces.     By  one  of  these 

forces,  the  moving  body  tends  to  fly  off  in  a  straight  line,  while 
by  the  other  it  is  drawn  toward  the  center,  and  thus  it  is  made 
to  revolve,  or  move  round  in  a  circle. 

197.  The  force  by  which  a  body  tends  to  go  off  in  a  straight 
line,  is  called  the  centrifugal  force  ;  that  which  keeps  it  from 
flying  away,  and  draws  it  toward  the  center,  is  called  the  cen- 
tripetal force. 

198.  Bodies  moving  in  circles  are  constantly  acted  upon  by 
these  two  forces.     If  the  centrifugal  force  should  cease,  the 
moving   body  would   no  longer   perform  a   circle,  but  would 
approach  the  center  of  its  own  motion.     If  the  centripetal  force 
should  cease,  the  body  would  instantly  begin  to  move  off  in  a 
straight  line,  this  being,  as  we  have  explained,  the  direction 
which  all  bodies  take  when  acted  on  by  a  single  force. 

195.  Expla'n  Fi2.  20.  and  show  on  what  principle  the  rider  meets  his  horse  after 
the  leap?  196.  What  is  circular  motion?  How  is  this  motion  produced?  197. 
What  is  the  centrifugal  force?  What  is  the  centripetal  force?  19S.  Suppose  the 
centrifugal  force  should  cease,  in  what  di'  ection  would  the  body  move  1  Suppose 
the  centripetal  force  should  cease,  where  would  the  body  go  ? 


46 


CIRCULAR   MOTION. 


FIG.  21 


Centrifugal  Force. 


199.  Suppose  a  cannon-ball, 
Fig'.  21,  tied  with  a  string  to 
the  center  of  a  slab  of  smooth 
marble,    and    suppose   an    at- 
tempt be  made  to  push   this 
ball  with  the  hand  in  the  di- 
rection of  b  ;  it  is  obvious  that 
the  string  would  prevent  its 
going  to  that  point ;  but  would 
keep  it  in  the  circle.     In  this 
case  the  string  is  the  centripe- 
tal force. 

200.  Now  suppose  the  ball 
to  be  kept  revolving  with  ra- 
pidity, its  velocity  and  weight 
would    cause    its     centrifugal 

force ;  and  if  the  string  were  cut,  when  the  ball  was  at  the 
point  c,  for  instance,  this  force  would  carry  it  off  in  the  line 
toward  b. 

The  greater  the  velocity  with  which  a  body  moves  round  in 
a  circle,  the  greater  will  be  the  force  with  which  it  would  tend 
to  fly  off  in  a  right  line. 

Thus,  when  one  wishes  to  sling  a  stone  to  the  greatest  dis  • 
tance,  he  makes  it  whirl  round  with  the  greatest  possible 
rapidity,  before  he  lets  it  go.  Before  the  invention  of  other 
warlike  instruments,  soldiers  threw  stones  in  this  manner,  with 
great  force  and  dreadful  effects. 

201.  The  line  about  which  a  body  revolves,  is  called  its  axis 
of  motion.      The  point  round  which  it  turns,  or  on  which  it 
rests,  is  called  the  center  of  motion.     In  Fig.  21,  the  point  c/,  to 
which  the  string  is  fixed,  is  the  center  of  motion.     In  the  spin 
ning-top,  a  line  through  the  center  of  the  handle  to  the  poirt 
on  which  it  turns,  is  the  axis  of  motion. 

In  the  revolution  of  a  wheel,  that  part  which  is  at  the  greatest 
distance  from  the  axis  of  motion,  has  the  greatest  velocity,  and, 
consequently,  the  greatest  centrifugal  force. 

202.  Suppose  the  wheel,  Fig.  22,  to  revolve  a  certain  number 
of  times  in  a  minute,  the  velocity  of  the  end  of  the  arm  at  the 
point   a,  would  be  as  much  greater  than  its  middle  at  the 
point  6,  as  its  distance  is  greater  from  the  axis  of  motion,  because 

199.  What  constitutes  the  centrifugal  force  of  the  body  moving  round  in  a  circle  1 
200.  How  is  this  illustrated  ?  201.  What  is  the  axis  of  motion  ]  What  is  the  centei 
of  motion?  Give  illustrations?  202.  What  part  of  a  revolving  wheel  has  the  great 
est  centrifugal  force  ? 


CENTER    OF    GRAVITY. 


4T 


Revolving  Wheel. 


it  moves  in  a  larger  circle,  and 
consequently  the  centrifugal 
force  of  the  rim  c,  would,  in  like 
manner,  be  as  its  distance  from 
the  center  of  motion. 

203.  Large  wheels,  which  are 
designed  to  turn  with  great  ve- 
locity, must,  therefore,  be  made 
with     corresponding     strength, 
otherwise  the   centrifugal  force 
will  overcome  the  cohesive  attrac- 
tion, or  the  strength  of  the  fast- 
enings, in  which  case  the  wheel 
will  fly  in  pieces.     This  some- 
time^ happens  to  the   large  grindstones  used  in  gun  factories, 
and  the  stone  either  flies  away  piecemeal,  or  breaks  in  the  mid- 
dle, to  the  great  danger  of  the  workmen. 

204.  Were  the  diurnal  velocity  of  the  earth  about  seventeen 
times  greater  than  it  is,  those  parts  at  the  greatest  distance  from 
its  axis  would  begin  to  fly  off  in  straight  lines,  as  the  water  does 
from  a  grindstone  when  it  is  turned  rapidly. 

CENTER.    OF    GRAVITY. 

205.  The  center  of  gravity,  in  any  body  or  system  of  bodies, 
is  that  point  upon  which   the  body,  or  system  of  bodies,   acted 
upon  only  by  gravity,  will  balance  itself  in  all  positions. 

206.  The   center  of  gravity,  in  a 
wheel  made  entirely  of  wood,  and  of 
equal  thickness,  would  be  exactly  in 
its  center  of  motion.     But  if  one  side 
of  the  wheel  were  made  of  iron,  and 
the  other  part  of  wood,  its  center  of 
gravity  would  be  changed  to   some 
point,  aside  from  the  center  of  the 
wheel. 

207.  Thus,  the  center  of  gravity 
in  the  wooden  wheel,  Fig.  23,  is  at 
the  axis  on  which  it  turns ;  but  were 
the  arm  a,  of  iron,  its  center  of  mo- 


no .  23. 


Center  of  Gravity. 


203.  Why  must  large  wheels,  turning  with  great  velocity,  be  strongly  marie  ?  204. 
vVhat  would  be  the  "consequence,  were  the  velocity  of  the  earth  seventeen  times 
greater  than  it  is  ?  205.  Where  is  the  center  of  gravity  in  a  body!  206  Where  is 
the  center  of  gravity  in  awheel  made  of  wood?  If  one  side  is  made  of  wood,  and  the 
other  of  iron,  where  is  the  center  ?  207.  Is  the  center  of  motion  and  of  gravity 
alwayg  the  same"? 


48 


CENTER   OF    GRAVITY. 


tion  and  of  gravity  would  no  longer  be  the  same,  but  while  the 
center  of  motion  remained  as  before,  the  center  of  gravity  would 
falf  to  the  point  a.  Thus  the  center  of  motion  and  of  gravity, 
though  often  at  the  same  point,  are  not  always  so. 

When  a  body  is  shaped  irregularly,  or  there  are  two  or  more 
bodies  connected,  the  center  of  gravity  is  the  point  on  which 
they  will  balance  without  falling. 


FIG.  24. 


FIG.  25. 


208.  If  the  two  balls  A  and  B,  Fig.  24,  weigh  each  four 
pounds,  the  center  of  gravity  will  be  a  point  on  the  bar  equally 
distant  from  each. 

But  if  one  of  the  balls  be  heavier  than  the  other,  then  the 
center  of  gravity  will,  in  proportion,  approach  the  larger  ball. 
Thus,  in  Fig.  25,  if  C  weighs  two  pounds,  and  D  eight  pounds, 
this  center  will  be  four  times  the  distance  from  C  that  it  is 
from  D. 

209.  In  a  body  of  equal  thickness,  as  a  board,  or  a  slab  of 
marble,  but  otherwise  of  an  irregular  shape,  the  center  of  gravity 
may  be  found  by  suspending  it,  first  from  one  point,  and  then 
from  another,  and  marking,  by  means  of  a  plumb-line,  the  per- 
pendicular ranges  from  the  point  of  suspension.     The  center  of 
gravity  will  be  the  point  where  these  two  lines  cross  each  other. 


FIG.  26. 


FIG.  27. 


FIG.  28. 


Finding  the  Center  of  Gravity. 

Thus,  if  the  irregular  shaped  piece  of  board,  Fig.  26,  be  sus- 
pended by  making  a  hole  through  it  at  the  point  A,  and  at  the 

208.  When  two  bodies  are  connected,  as  by  a  bar  between  them,  where  is  the  cen- 
ter of  gravity  ?  209.  In  a  board  of  irregular  shape,  by  what  method  is  the  center  of 
gravity  found  1 


CENTER    OF    GRAVITY. 

same  point  suspending  the  plumb-line  C, 

v  ill  hang  in  the  position  represented  in 

marked  this  line  across  the  board,  let  it  be  suspfc^ggMg/iiu   in 

the  position  of  Fig.  27,  and  the  perpendicular  line  again  marked. 

The  point  where  these  lines  cross,  is  the  center  of  gra\ity,  as 

seen  by  Fig.  28. 

210.  Importance  of  the  subject. — It  is  often  of  great  conse- 
quence, in  the  concerns  of  life,  that  the  subject  of  gravity  should 
be  well  considered,  since  the  strength  of  buildings,  and  of  ma- 
chinery, often  depends  chiefly  on  the  gravitating  point. 

Common  experience  teaches,  that  a  tall  object,  with  a  nar- 
row base,  or  foundation,  is  easily  overturned  ;  but  common 
experience  does  not  teach  the  reason,  for  it  is  only  by  under- 
standing principles,  that  practice  improves  experiment. 

211.  An  upright  object  will  fall  to  the  ground,  when  it  leans 
so  much  that  a  perpendicular  line  from  its  center  of  gravity  falls 
beyond  its  base.     A  tall   chimney,   therefore,  with  a  narrow 
foundation,  such  as  are  commonly  built  at  the  present  day,  will 
fall  with  a  very  slight  inclination. 

212.  Now,  in  falling,  the  center  of  gravity  passes  through 
the  part  of  a  circle,  the  center  of  which  is  at  the  extremity  of 
the  base  on  which  the  body  stands.     This  will  be  comprehended 
by  Fig.  29. 

Suppose  the  figure  FIG  29.  FIG.  so. 

to  be  a  block  of  mar- 
ble, which  is  to  be 
turned  over,  by  lift- 
ing at  the  corner  A, 
the  corner  B  would 
be  the  center  of  its 
motion,  or  the  point 

on  which  it  would  turn.  The  center  of  gravity,  C,  would,  there- 
fore, describe  the  part  of  a  circle,  of  which  the  corner,  B,  is  the 
center. 

213.  It  will  be  found  that  the  greatest  difficulty  in  turning 
over  a  square  block  of  marble,  is  in  first  raising  up  the  center 
of  gravity,  for  the  resistance  will  constantly  become  less,  in  pro- 
portion as  the  point  approaches  a  perpendicular  line  over  the 
CDrner  B,  which,  having  passed,  it  will  fall  by  its  own  gravity. 


210  Why  is  findjns  the  center  of  gravity  of  importance  ?  211.  In  what  direction 
must  the  center  of  gravity  be  from  the  outs  de  of  the  base,  before  the  object  will  fall^ 
212.  In  falling,  the  center  of  gravity  passes  through  part  of  a  circle  :  where  is  the  cen- 
ter of  this  circle  1  213.  In  turning  over  a  body  why  does  the  force  required  con 
•tantly  become  less  and  less  ? 

3 


60  CENTER    OF    GRAVITT. 

The  difficulty  in  turning  over  a  body  of  particular  form,  will 
be  more  strikingly  illustrated  by  the  figure  of  a  triangle,  or  low 
pyramid. 

214.  In  Fig.  30,  the  center  of  gravity  is  so  low,  and  the  base 
so  broad,  that  in  turning  it  over,  a  great  proportion  of  its  whole 
weight  must  be  raised.     Hence  we  see  the  firmness  of  the  pyra- 
mid in  theory,  and  experience  proves  its  truth  ;  for  buildings  are 
found  to  withstand  the  effects  of  time,  and  the  commotions  of 
earthquakes,  in  proportion  as  they  approach  this  figure. 

The  most  ancient  monuments  of  the  art  of  building,  now 
standing,  the  pyramids  of  Egypt,  are  of  this  form. 

215.  Movement  of  a  Ball. — When  a  ball  is  rolled  on  a  hori- 
zontal plane,  the  center  of  gravity  is  not  raised,  but  moves  in  a 
straight  line,  parallel  to  the  surface  of  the  plane  on  which  it 
rolls,  and  is  consequently  always  directly  over  its  center  of 
motion. 

216.  Suppose,  Fig.  31,  A  is  the  FIG.  31. 
plane  on  which  the  ball  moves,  B 

the  line  on  which  the  center  of  grav- 
ity moves,  and  C  a  plumb-line,  show- 
ing that  the  center  of  gravity  must 
always  be  over  the  center  of  motion, 
when  the  ball  moves  on  a  horizontal 
plane — then  we  shall  see  the  reason 
why  a  ball  moving  on  such  a  plane, 
will  rest  with  equal  firmness  in  any 
position,  and  why  so  little  force  is  required  to  set  it  in  motion. 
For  in  no  other  figure  does  the  center  of  gravity  describe  a 
horizontal  line  over  that  of  motion,  in  whatever  direction  the 
body  is  moved. 

217.  If  the  plane  is  inclined  downward,  the  ball  is  instantly 
thrown  into  motion,  because  the  center  of  gravity  then  falls  for- 
ward of  that  of  motion,  or  the  point  on  which  the  ball  rests. 

218.  This  is  explained  by  Fig.  32,  where  A  is  the  point  on 
which  the  ball  rests,  or  the  center  of  motion,  13  the  perpendicular 
line  from  the  center  of  gravity,  as  shown  by  the  plumb- weight  C. 

219.  If  the  plane  is  inclined  upward,  force  is  required  to 
move  the  ball  in  that  direction,  because  the  center  of  gravity 

214.  Why  is  there  less  force  required  to  overturn  a  cube,  or  square,  than  a  pyra- 
m:d  of  the  same  weight?  215.  When  a  ball  is  rolled  on  a  horizontal  plane,  in  what 
direction  does  the  center  of  gravity  move  1  Explain  Fig.  31.  21t>.  Why  does  a  ball 
on  a  horizontal  plane  ret;t  equally  well  in  all  positions  1  Why  does  it  move  with  little 
force?  217.  If  the  plane  is  inclined  downward,  why  does  the  ball  roll  in  that  direc- 
tion 1  218.  Explain  Fijj.  32.  2|.9.  Why  is  force  required  to  move  a  hall  UD  an  in- 
e.inedpla««1 


CENTER    OP    GRAVITY. 


51 


Cii 


FIG.  33. 


then  falls  behind  that  of  motion,  and 
therefore  this  point  has  to  be  constantly 
lifted.  This  is  also  shown  by  Fig.  32, 
only  considering  the  ball  to  ba  moving 
up  the  inclined  plane,  instead  of  down  it. 
From  these  principles,  it  will  be  read- 
ily understood  why  so  much  force  is 
required  to  roll  a  heavy  body,  as  a 
hogshead  of  sugar,  for  instance,  up  an 
inclined  plane.  The  center  of  gravity 
falling  behind  that  of  motion,  the  weight 

is  constantly  acting  against  the  force  employed  to  raise  the 
body. 

220.  Illustration  by  Blocks. — One  of  the  best  illustrations  of 
this  subject  may  be  made  by  a  number  of  square  blocks  of  wood, 
placed  on  each  other,  as  in  Fig.  33,  forming  a 

leaning  tower.  Where  five  blocks  are  placed 
in  this  position,  the  point  of  gravity  is  near  the 
center  of  the  third  block,  and  is  within^  the 
base,  as  shown  by  the  plumb-line.  But  on 
adding  another  block,  the  gravitating  point 
falls  beyond  the  base,  and  the  whole  will  now 
fall  by  its  own  weight. 

221.  The  student  having  such  blocks,  (and 
they  may  be  picked   up  about  any  joiner's 
shop,)  will  convince  himself,  that  however  care- 
fully his  leaning  tower  is  laid  up,  it  will  not 
stand  when  the  center  of  gravity  falls  an  inch 
or  two  beyond  the  support. 

222.  We  may  learn,  from  these  compari- 
sons, that  it  is  more  dangerous  to  ride  in  a 
high  carriage  than  a  low  one,  in  proportion  to 
the  elevation  of  the  vehicle,  and  the  nearness 

of  the  wheels  to  each  other,  or  in  proportion  to  the  narrowness 
of  the  base,  and  the  height  of  the  center  of  gravity.  A  load  of 
hay,  Fig.  34,  upsets  where  one  wheel  rises  but  little  above  the 
other,  because  it  is  broader  on  the  top  than  the  distance  of  the 
wheels  from  each  other ;  while  a  load  of  stone  is  very  rarely 
turned  over,  because  the  center  of  gravity  is  near  the  earth,  and 
its  weight  between  the  wheels,  instead  of  being  far  above  them. 

220.  Why  is  a  body,  shaped  like  Fig.  33,  easily  thrown  down  ?  Hence,  in  riding 
in  a  carriage,  how  ;s  the  danger  of  upsetting  proportioned?  Explain  Fig.  33.  221. 
How  may  the  point  of  gravity  be  found  by  means  of  a  number  of  square  blocks  1 
222.  Why  will  a  load  of  hay  upset  more  readily  than  one  of  stone! 


Leaning  Tower. 


52 


CENTER    OF    GRAVITY. 


Load  of  Hay. 


223.  Center  of  gravity  in  man. — :  FIQ- 
In  man  the  center  of  gravity  is  be- 
tween the  hips,  and  hence,  were  his 

feot  tied  together,  and  his  arms  tied 
to  his  sides,  a  very  slight  inclination 
of  his  body  would  carry  the  perpen- 
dicular of  his  center  of  gravity  be- 
yond the  base,  and  he  would  fall. 
But  when  his  limbs  are  free  to  move, 
he  widens  his  base,  and  changes  this 
center  at  pleasure,  by  throwing  out 
his  arms,  as  circumstances  require. 

When  a  man  runs,  he  inclines  for- 
ward, so  that  the  center  of  gravity 
may  hang  before  his  base,   and  in 
this  position  he  is  obliged  to  keep  his  feet  constantly  advancing, 
otherwise  he  would  fall  forward. 

A  man  standing  on  one  foot,  can  not  throw  his  body  forward 
without,  at  the  same  time,  throwing  his  other  foot  backward,  in 
order  to  keep  the  center  of  gravity  within  the  base. 

224.  A  man,  therefore,  standing  with  his  heels  against  a  per- 
pend'cular  wall,  can  not  stoop  forward  without  falling,  because 
the  wall  prevents  his  throwing  any  part  of  his  body  backward. 
A  person,  little  versed  in  such  things,  agreed  to  pay  a  certain 
sum  of  money  for  an  opportunity  of  possessing  himself  of  double 
the  sum,  by  taking  it  from  the  floor  with  his  heels  against  the 
wall.     The  man,  of  course,  lost  his  money,  for  in  such  a  posture, 
one  can  hardly  reach  lower  than  his  own  knee. 

225.  The  base  on  which  a  man  is  supported,  in  walking  or 
standing,  is  his  feet,  and  the  space  between  them.     By  turning 
the  toes  out,  this  base  is  made  broader,  without  taking  much 
from  its  length,  and  hence  persons  who  turn  their  toes  outward, 
not  only  walk  more  firmly,  but  more  gracefully,  than  those  who 
turn  them  inward. 

226.  In  consequence  of  the  upright  position  of  man,  he  is 
constantly  obliged  to  employ  some  exertion  to  keep  his  balance. 
This  seems  to  be  the  reason  why  children  learn  to  walk  with  so 
much  difficulty ;  for  after  they  have  strength  to  stand,  it  re- 
quires considerable  experience  so  to  balance  the  body  as  to  set 
one  foot  before  the  other  without  falling. 

,  223  Where  is  the  center  of  man's  gravity  ?  Why  will  a  man  fall  with  a  slight  in- 
clination, when  his  feet  and  arms  are  tied  ?  224.  Why  can  not  one  who  stands  with 
his  heels  against  a  wall  stoop  forward  ?  225.  Why  does  a  person  walk  most  firmly, 
•who  turns  his  toes  outward!  226.  Why  does  not  a  child  walk  as  soon  as  he  can 
stand  7 


CENTER    OF    GRAVITY. 


53 


FIG.  35. 


227.  By  experience  in  the  art  of  balancing,  or  of  keeping  the 
center  of  gravity  in  a  line  over  the  base,  men  sometimes  perform 
things  that,  at  first  sight,  appear   altogether  beyond  human 
pow^r,  such  as  dining  with  the  table  and  chair  -standing  on  a 
single  rope,  dancing  on  a  wire,  &c. 

228.  Illustration  by  Tr$es. — No  form,  under  which  matter 
exists,  escapes  the  general  law  of  gravity,  and  hence  vegetables, 
as  well  as  animals,  are  formed  with  reference  to  the  position  of 
this  center,  in  respect  to  the  base. 

It  is  interesting,  in  reference  to  this  circumstance,  to  observe 
how  exactly  the  tall  trees  of  the  forest  conform  to  this  law. 

The  pine,  which  grows  a  hundred  feet  high,  shoots  up  with 
as  much  exactness,  with  respect  to  keeping  its  center  of  gravity 
within  the  base,  as  though  it  had  been  directed  by  the  plumb- 
line  of  a  master  builder.  Its  limbs  toward  the  top  are  sent  off 
in  conformity  to  the  same  law ;  each  one  growing  in  respect  to 
the  other,  so  as  to  preserve  a  due  balance  between  the  whole. 

229.  SHEPHERDS  OF  LANDES. — Men,  as  already  noticed,  by 
practice  in  the  art  of  balancing, 

pei-form  feats  which  are  won- 
derful to  all  beholders.  The 
shepherds  of  Landes,  in  the 
south  of  France,  are  perhaps 
the  only  people  who  apply  this 
art  to  the  common  business  of 
life.  These  men  walk  on  stilts 
from  four  to  five  feet  high ;  and 
their  children,  when  quite 
young,  who  are  intended  to 
take  the  places  of  their  fathers 
as  shepherds,  are  taught  this 
art  in  order  to  qualify  them  for 
business. 

To  strangers,  passing  then- 
district,  these  men  cut  a  figure 
at  once  ludicrous  and  surpris- 
ing. Fig.  35.  But  it  is  for 
their  own  convenience  that  this 
singular  custom  has  been  adopt- 
ed, for  by  this  means  the  feet 
are  kept  out  of  the  water  which  Shepherd  of  Land 


227.  In  what  does  the  art  of  balancing,  or  walking  on  a  rope,  consist  ?  228.  What 
is  observed  in  the  growth  of  the  trees  of  the  forest,  in  respect  to  the  laws  of  gravity  1 
229.  What  principle  is  involved  in  Fig.  35. 


54  CENTER    OF   INERTIA. 

covers  their  land  in  the  winter,  and  from  the  heated  sand  in  the 
summer.  Besides  these  comforts,  the  sphere  of  vision  over  a 
flat  country  is  materially  increased  by  the  elevation,  so  that  the 
shepherd  can  see  his  flock  at  a  much  greater  distance  than  from 
the  ground. 

By  -habit,  it  is  said  these  men  acquire  the  art  of  balancing 
themselves  so  perfectly  as  to  run,  jump,  and  dance  on  these 
stilts  with  perfect  ease.  They  walk  with  surprising  quickness, 
so  that  footmen  have  to  do  their  best  to  keep  up  with  them. 

CENTER   OF   INERTIA,    OR    INACTIVITY. 

230.  It  will  be  remembered  that  inertia  (22)  is  one  of  the 
inherent,  or  essential  properties  of  matter,  and  that  it  is  in  con- 
sequence of  this  property,  when  bodies  are  at  rest,  that  they  never 
move  without  the  application  of  force,  and  when  once  in  motion, 
that  they  never  cease  moving  without  some  external  cause.  (27.) 

231.  Now,  inertia,  though  like  gravity,  it  resides  equally  in 
every  particle  of  matter,  must  have,  like  it,  a  center  in  each  par- 
ticular body,  and  this  center  is  the  same  with  that  of  gravity. 

232.  In  a  bar  of  iron,  six  feet  long  and  two  inches  square, 
this  center  is  just  three  feet  from  each  end,  or  exactly  in  the 
middle.     If,  therefore,  the  bar  is  supported  at  this  point,  it  will 
balance  equally,  and  because  there  are  equal  weights  on  both 
ends,  it  will  not  fall. 

Now  suppose  a  bar  should  be  raised  by  raising  up  the  center 
of  gravity,  then  the  inertia  of  all  its  parts  would  be  overcome 
equally  with  that  of  the  middle.  The  center  of  gravity  is, 
therefore,  the  center  of  inertia. 

233.  But,   suppose  FIQ  36 
the  same  bar  of  iron, 

whose  inertia  was  over- 
come by  raising  the 
center,  to  have  balls  of 

different    weights     at-  Center  of  Inertia. 

tacned  to  its  ends; 
then  the  center  of  inertia  would  no  longer  remain  in  the  middle 
of  the  bar,  but  would  be  changed  to  the  point  A,  Fig.  36,  so 
that,  to  lift  the  whole,  this  point  must  be  raised,  instead  of  the 
middle,  as  before. 


230.  \V1iat  effect  does  inertia  exert  on  bodies  at  rest  ?  What  effect  does  it  have  on 
bodies  in  motion  ?  231.  Is  the  center  of  inertia,  and  that  of  gravity,  the  same  1  232. 
Where  is  the  center  of  inertia  in  a  body,  or  a  system  of  bodies  1  233.  Why  is  the 
point  of  inertia  changed,  by  fixing  different  weights  to  the  ends  of  the  iron  bar  7 


EQUILIBRIUM. 


EQUILIBRIUM. 

234.  When  two  forces  counteract,  or  balance  each  other,  tftey 
are  said  to  be  in  equilibrium. 

235.  It  is  not  necessary  for  this  purpose  that  the  weights 
opposed  to  each  other  should  be  equally  heavy,  for  we  have 
just  seen  that  a  small  weight,  placed  at  a  distance  from  the 
center  of  inertia,  will  balance  a  large  one  placed  near  it.     To 
produce  equilibrium,  it  is  only  necessary  that  the  weights  on 
each  side  of  the  support  should  mutually  counteract  each  other, 
or  if  set  in  motion,  that  their  momenta  should  be  equal. 

236.  A  pair  of  scales  are  in  equilibrium  when  the  beam  is 
in  a  horizontal  position. 

To  produce  equilibrium  in  solid  bodies,  therefore,  it  is  only 
necessary  to  support  the  center  of  inertia,  or  gravity. 

237.  If  a  body,  or  sev- 
eral bodies,  connected,  be  FIG.  37. 
suspended  by  a  string,  as 

in  Fig.  37,  the  point  of 
support  is  always  in  a  per- 
pendicular line  above  the 
center  of  inertia.  The 
plumb-line,  D,  cuts  the  bar 
connecting  the  two  balls  at 
this  point.  Were  the  two 
weights  in  this  figure  equal, 

it  is  evident  that  the  hook,  Equilibrium. 

or  point  of  support,  must 

be  in  the  middle  of  the  string,   to  preserve  the   horizontal 
position. 

238.  When  a  man  stands  on  his  right  foot,  he  keeps  himself 
in  equilibrium,  by  leaning  to  the  right,  so  as  to  bring  his  center 
of  gravity  in  a  perpendicular  line  over  the  foot  on  which  he 
stands. 


CURVILINEAR,    OR    BENT    MOTION. 


239.  We  have  seen  that  a  single  force  acting  on  a  body,  (183,) 
drives  it  straight  forward,  and  that  two  forces  acting  crosswise, 
drive  it  midway  between  the  two,  or  give  it  a  diagonal  direc- 
tion, (190.) 

234.  What  is  meant  by  equilibrium?  235.  To  produce  equilibrium,  must  the 
weights  be  equal  1  236.  When  is  a  pair  of  scales  in  equilibrium  ?  237.  When  a  body 
is  suspended  by  a  string,  where  must  the  support  be  with  respect  to  the  point  ol  in- 
ertia J  239.  What  is  meant  by  curvilinear  motion  1 


50  CURVILINEAR    MOTION. 

Curvilinear  motion  differs  from  both  these ;  the  direction  of 
the  body  being  neither  straight  forward  nor  diagonal,  but  through 
a  line  which  is  curved. 

This  kind  of  motion  may  be  in  any  direction  ;  but  when  it  is 
produced  in  part  by  gravity,  its  direction  is  always  toward  the 
earth. 

239.  A  stream  of  water  from  an  aperture  in  the  side  of  a 
vessel,  as  it  falls  toward  the  ground,  is  an  example  of  a  curved 
line ;  and  a  body  passing  through  such  a  line,  is  said  to  have 
curvilinear  motion.     Any  body  projected  forward,  as  a  cannon- 
ball,  or  rocket,  falls  to  the  earth  in  a  curved  line. 

240.  It  is  the  action  of  gravity  acioss  the  course  of  the  stream, 
or  the  path  of  the  ball,  that  bends  it  downward  and  makes  it 
form  a  curve.     The  motion  is,  therefore,  the  result  of  two  forces, 
that  of  ] -rojection,  and  that  of  gravity. 

241.  In  jets  of  water,  the  shape  of  the  curve  will  depend  on 
the  velocity  of  the  stream.     When  the  pressure  of  the  water  is 
great,  the  stream,  near  the  vessel,  is  nearly  horizontal,  because 
its  velocity  is  in  proportion  to  the  pressure.     When  a  ball  first 
leaves  the  cannon,  it  describes  but  a  slight  curve,  because  its 
projectile  velocity  is  then  greatest. 

242.  The  curves  described,  by  jets  of  water  under  different 
degrees  of  pressure,  are  readily  illustrated  by  tapping  a  tall 
vessel  in  several  places,  one  above  the  other. 

243.  The  action  of  gravity  being  always  the  same,  the  shape 
of  the  curve  described  must  depend  on  the  velocity  of  the  mov- 
ing body ;  but  whether  the  projectile  force  be  great  or  small, 
the  moving  body,  if  thrown  horizontally,  will  reach  the  ground 
from  the  same  height  in  the  same  time. 

This,  at  first  thought,  would  seem  improbable ;  for,  without 
consideration,  most  persons  would  assert,  that,  if  two  cannons 
were  fired  from  the  same  spot  at  the  same  instant,  and  in  the  same 
direction,  one  of  the  balls  falling  half  a  mile,  and  the  other  a 
mile  distant,  that  the  ball  which  went  to  the  greatest  distance 
would  take  the  most  time  in  performing  its  journey. 

244.  But  it  must  be  remembered,  that  the  projectile  force 
does  not  in  the  least  interfere  with  the  force  of  gravity.     A  ball 


239.  What  are  examplps  of  this  kind  of  motion?  240.  What  two  forces  produce 
this  motion?  241  On  what  does  the  shape  of  the  curve  depend  ?  242.  How  are  the 
curves  described  by  jets  of  water  illustrated?  243  What  difference  is  there  in  re- 
spect to  the  time  taken  by  a  body  to  reach  the  ground,  whether  the  curve  be  grf  at  or 
small  ?  244.  Why  do  bod  es,  forming  different  curves  from  the  same  height,  reach 
the  srrouml  at  the  same  time  ?  Suppose  two  balls,  one  flying  at  the  rate  of  a  thousand, 
and  the  other  at  the  rate  of  a  hundred  feet  per  second,  which  would  descend  most 
during  the  second  7 


CURVILINEAR    MOTION".  57 

flying  horizontally,  at  the  rate  of  a  thousand  feet  per  second,  is 
attracted  downward  with  precisely  the  same  force  as  one  flying 
only  a  hundred  feet  per  second,  and  must,  therefore,  descend 
the  same  distance  in  the  same  time. 

245.  The  distance  to  which  a  ball  will  go,  depends  on  the 
force  of  impulse  given  it  the  first  instant,  and,  consequently,  on 
its  projectile  velocity.     If  it  moves  slowly,  the  distance  will  be 
short;  if  more  rapidly,  the  space  passed  over  will  be  greater. 
It  makes  no  difference,  then,  in  respect  to  the  descent  of  the 
ball,  whether  its  projectile  motion  be  fast  or  slow,  or  whether  it 
moves  forward  at  all. 

246.  Falling  of  Cannon  Balls. — This  may  be  shown  by  ex- 
periment.    Suppose  a  cannon  to  be  loaded  with  a  ball,  and 
placed  on  the  top  of  a  tower,  at  such  a  height  from  the  ground, 
that  it  would  take  just  four  seconds  for  the  ball  to  descend  from 
it  to  the  ground,  if  let  fall  perpendicularly.     Now,  suppose  the 
cannon  to  be  fired  in  an  exact  horizontal  direction,  and,  at  the 
same  instant,  the  ball  to  be  dropped  toward  the  ground.     They 
will  both  reach  the  ground  at  the  same  instant,  provided  its 
surface  be  a  horizontal  plane  from  the  foot  of  the  tower  to  the 
place  where  the  projected  ball  strikes. 

247.  Demonstration. — This  is  demonstrated  by  Fig.  38,  where 
A  is  the  cannon  from  which  the  ball  is  to  be  fired,  a  the  ver- 
tical line  of  the  descending  ball,  A,  B,  1,  a,  the  parallelogram 
through  which  the  ball  passes  during  the  first  second. 

Now  the  ball  dropped  in  the  vertical  direction,  will  descend 
16  feet  the  first  second,  increasing  its  velocity  according  to  the 
law  of  falling  bodies  already  explained.  Meantime  the  pro- 
jected ball  passing  through  the  diagonal  of  the  upper  parallelo- 
gram, will  arrive  at  1,  while  the  other  falls  to  a.  During  the 
next  second  the  vertical  ball  will  fall  to  6,  while  the  other,  in 
consequence  of  its  projectile  force,  will  pass  through  the  diago- 
nal of  the  parallelogram  6,  2,  C,  A. 

The  same  laws  of  descent  being  continued,  it  is  obvious,  that 
the  two  balls  will  reach  d,  4  at  the  same  instant. 

248.  From  these  principles  it  may  be  inferred,  that  the  hoi- 
izontal  motion  of  a  body  through  the  air,  does  not  interfere 
with  its  gravitating  motion  toward  the  earth,  and,  therefore, 


245.  Does  it  make  any  difference  in  respect  to  the  descent  of  the  ball,  whether  it 
has  a  projectile  motion  or  not  1  246.  Suppose,  then,  one  hall  be  fired  from  a  cannon, 
and  another  let  fall  from  the  same  height  at  the  same  instant,  would  they  both  reach 
the  ground  at  the  same  time  1  247.  Explain  Fig.  33.  showing  the  reason  why  the 
two  bails  will  reach  the  sround  at  the  same  time.  Why  does  the  ball  approach  the 
earih  more  rapidly  in  the  last  part  of  the  curve  than  in  the  first  parti  248.  What  ia 
t:ie  inference  from  these  princ  pies  ? 

3* 


58 


CURVILINEAR    MOTION. 


Path  of  a  Cannon  Ball. 


that  a  rifle-ball,  or  any  other  body  projected  horizontally,  will 
reach  the  ground  in  the  same  period  of  time  as  one  that  is  let 
fall  perpendicularly  from  the  same  height. 

249.  The  two  forces  acting  on  bodies  which  fall  through 
curved  lines,  are  the  same  as  the  centrifugal  and  centripetal 
forces,  already  explained ;  the  centrifugal,  in  case  of  the  ball, 
being  caused  by  the  powder — the  centripetal,  being  the  action 
of  gravity,  (199.) 

250.  Now  the  space  through  which  a  cannon-ball,  or  any 
other  body,  can  be  thrown,  depends  on  the  velocity  with  which 
it  is  projected ;  for  the  attraction  of  gravitation,  and  the  resist- 
ance of  the  air,  acting  perpetually,  the  time  which  a  projectile 
can  be  kept  in  motion  through  the  air  is  only  a  few  moments. 

Perpetual  Revolution.— If  the  projectile  be  thrown  from  an 
elevated  situation,  it  is  plain  that  it  would  strike  at  a  greater 
distance  than  if  thrown  on  a  level,  because  it  would  remain 
longer  in  the  air.  Every  one  knows  that  he  can  throw  a  stone 
to  a  greater  distance  when  standing  on  a  steep  hill,  than  when 
standing  on  the  plain  below. 

251.  Suppose  the  circle,  Fig.  39,  to  be  the  earth,  and  A,  a 
high  mountain  on  its  surface.     Suppose  that  this  mountain 

249.  What  is  the  force  called  which  throws  a  ball  forward  ?  What  is  that  called 
which  brings  it  to  the  ground  1  250.  On  what  does  the  distance  to  which  a  projected 
body  may  be  thrown,  depend  ?  Why  does  the  distance  depend  on  the  velocity  ? 
251.  Suppose  the  velocity  of  a  cannon-ball  shot  from  a  mountain  50  miles  high  to  be 
ten  times  its  usual  rate,  where  would  it  stop  ? 


GUNNERY. 


reaches  above  the  atmos-  FIG.  39. 

phere,  or  is  fifty  miles 
high,  then  a  cannon-ball 
might  perhaps  reach 
from  A  to  B,  a  distance 
of  eighty  or  a  hundred 
miles,  because  the  resist- 
ance of  the  atmosphere 
being  out  of  the  calcula- 
tion, it  would  have  noth- 
ing to  contend  with,  ex- 
cept the  attraction  of 
gravitation.  Ifj  then,  one 
degree  of  force,  or  veloc- 
ity, would  send  it  toB, 
another  would  send  it  to 
C ;  and  if  the  force  was 
increased  three  times,  it  would  fall  to  D,  and  if  four  times,  it 
would  pass  to  E.  If,  now,  we  suppose  the  force  to  be  about  ten 
times  greater  than  that  with  which  a  cannon-ball  is  projected, 
it  would  not  fall  to  the  earth  at  any  of  these  points,  but  would 
continue  its  motion  until  it  again  came  to  the  point  A,  the  place 
from  which  it  was  first  projected. 

252.  It  would  now  be  in  equilibrium,  the  centrifugal  force 
being  just  equal  to  that  of  gravity,  and,  therefore,  it  would  per- 
form another  and  another  revolution,  and  so  continue  to  revolve 
around  the  earth  perpetually. 

253.  It  is  these  two  forces  which  retain  the  heavenly  bodies 
in  their  orbits ;  and  in  the  case  we  have  supposed,  our  cannon- 
ball  would  become  a  little  satellite,  moving  perpetually  round 
the  earth. 


Perpetual  Revolution  of  a 


254.  LAW  AND  FORCE  OF  PROJECTILES. — Ever  since  the  dis- 
covery of  gunpowder,  the  laws  of  projectiles  have  been  studied 
with  attention,  as  being  of  importance  in  the  art  of  war.     Many 
learned  and  elaborate  works  have  been  published  on  the  sub- 
ject, but  our  limits  will  only  admit  the  insertion  of  a  few  of  the 
most  important  principle's  of  Gunnery. 

255.  A  projectile,  as  a  bullet  from  a  gun,  unless  it  has  a 

Explain  Fig.  39.  252.  When  would  this  ball  be  in  equilibrium  ?  Why  would  not 
the  force  of  gravity  ultimately  brin?  this  hall  to  the  earth  ?  253.  After  the  fiist  revo- 
lution, if  the  two  forces  continued  the  same,  would  not  the  motion  of  the  ball  be  per 
petuall  254.  Why  are  the  laws  of  projectiles  viewed  important?  255.  What  two 
forces  act  on  projectiles  7  What  is  the  path  of  a  projectile  called  7 


60 


GUNNERY. 


vertical  direction,  is  acted  on  by  two  forces,  that  of  projection, 
which  carries  it  forward,  and  that  of  gravity,  which  draws  it 
downward.  Its  path,  therefore,  is  a  curve,  called  a  parabola. 

256.  The  distance  to  which  the  ball  will  fly,  depends  on  the 
force  of  projection,  since,  if  its  direction  is  horizontal,  its  fall 
toward  the  earth  by  the  force  of  gravity  (250)  will  be  the  same, 
whether  its  velocity  be  great  or  small. 

257.  The  resistance  of  the  atmosphere,  is  the  great  impedi- 
ment to  the  effects  of  projectile  forces.     Thus  it  has  been  de- 
monstrated that  a  24-lb.  cannon-ball,  discharged  at  an  elevation 
of  45°,  and  at  the  velocity  of  2000  feet  per  second,  would,  in 
vacuo,  reach  the  horizon -distance  of  125,000  feet,  but  the  re- 
sistance of  the  air  limit's  its  range  to  7,300  feet. 

258.  VELOCITY  OF  THE  BALL. — There  are  several  methods 
of  computing  the  velocity  of  the  ball,  one  of  which  is  by  means 
of  the  Ballistic  pendulum.     This  is  a  thick,  heavy  block  of 
wood,  so  suspended  as  to  swing  freely  about  on  axis,  and  into 
this  the  ball  is  fired.     The  weight  of  the  ball,  and  that  of  the 
block  being  known,  the  velocity  is  found,  by  the  degrees  of  mo- 
tion given  to  the  pendulum,  which  is  accurately  measured  by 
machinery. 

259.  Recoil  of  the  Gun. — Another  method  of  finding  the 
velocity  of  the  ball,  is  by  means  of  the  recoil  of  the  gun.     This 
method  is  founded  on  the  supposition  that  the  explosive  force 
of  the  powder,  communicates  equal  quantities  of  motion  to  the 
gun  and  ball,  in  opposite  directions.     Hence,  by  suspending 
the  gun,  loaded  with  weights,  like  a  pendulum,  the  extent  of  its 
arc  of  vibration,  will  indicate  the  force  of  the  charge,  and  by 
knowing  the  weights  of  the  gun  and  ball,  its  velocity  is  indica- 
ted.    By  such  means    Dr.  Hutton  constructed  the  following 
table  :— 


POWDER. 

VELOCITY    PER    SECOND. 

DISTANCE. 

TIME    OF    FLIGHT. 

Ounces. 

Feet. 

Feet. 

Seconds. 

2 

800 

4100 

9 

4 

1230 

5100 

12 

8 

1640 

6000 

14|- 

12 

1680 

6700 

151 

260.  Experiment  shows  that  the  velocity  of  the  ball  increases 

256.  On  what  does  the  distance  of  a  projectile  depend  ?  257.  What  is  said  of  at- 
mospheric resistance  1  258.  What  is  the  construction  of  the  ballistic  pendulum  1 
859.  What  is  the  other  method  of  estimating  the  velocity  of  the  ball1? 


PERCUSSION    CAPS.  61 

with  the  charge,  to  a  certain  extent,  which  is  peculiar  to  each 
gun,  after  which  the  increase  diminishes  the  force,  until  the 
bore  is  quite  full. 

261.  The  greatest  velocity  of  a  ball  known,  is  about  2000 
feet  per  second,  and  this  from  a  cannon.     This  velocity  dimin- 
ishes, soon  after  it  leaves  the  gun. 

262.  Power  and  Destruction. — The  penetration  of  the  ball 
is  as  the  square  of  its  velocity.     Hence,  when  the  object  is 
merely  to  penetrate,  as  in  the  breaching  of  a  fortification,  the 
greatest  velocity  is  given.     But  in  naval  combats,  the  utmost 
velocity  is  not  the  most  injurious,  the  most  destructive  balls 
being  such  as  merely  pierce  the  ship's  sides. 

MANUFACTURE    OF    PERCUSSION    CAPS. 

263.  The  processes  by  which  percussion  caps  are  made  at  the 
establishment  of  Mr.  Mclntyre,  in  the  city  of  Hartford,  Ct.,  are 
as  follow : — 

264.  First. — The  copper  is  rolled  to  about  the  thickness  of 
stout  brown  paper,  and  is  then  cut  into  strips  three-fourths  of  an 
inch  wide,  and  several  yards  long.     The  end  of  such  a  strip 
being  placed  between  the  rollers  of  a  cutting  and  punching 
machine,  invented  for  this  purpose,  the  whole,  without  further 
attention,  is  cut  into  star-like  pieces  of  the  form  and  size  repre- 
sented by  Fig.  40,  A  being  the  piece  cut  out,  and  B,  the  ap- 
pearance of  the  strip  of  copper  after  the  operation. 

FIG.  40. 


First  shape  of  the  Copper. 

These  pieces  are  instantly  moved,  by  the  same  engine,  under 
the  punch,  by  which  they  are  driven  through  a  finely  creased 
die,  and  are  thus  formed  into  caps  which  fall  into  a  vessel 
below. 

These  stellate  pieces,  being  struck  by  the  punch  in  the  cen- 
ter, the  extremities  are  thus  brought  into  contact,  but  not 
joined,  so  that  the  caps  consist  of  four  portions  connected  at  the 
bottom,  like  the  four  quarters  of  an  orange  peel. 

260.  How  far  does  the  velocity  of  the  ball  increase  with  that  of  the  charge!  261. 
What  is  the  greatest  velocity  of  a  ball  7  262.  What  velocity  of  the  ball  is  most  de- 
structive ? 


62 


PERCUSSION    CAPS. 


When  the  caps  are  exploded  by.  the  hammer,  these  quarters 
open  and  thus  prevent  the  tearing  of  the  metal  which,  if  solid, 
would  be  apt  to  fly  into  fragments  and  thus  endanger  the 
eyes. 

265.  Second. — The  caps  are  next  placed  in  a  revolving  cylin- 
der containing  saw-dust,  by  which  they  are  made  clean  and 
bright. 

They  are  now  ready  to  receive  the  fulminating  powder,  the 
explosion  of  which  sets  fire  to  the  powder  in  the  gun-barrel. 

The  caps  are  now  placed,  a  handful  at  a  time,  on  a  sheet  of 
iron,  three  feet  long,  eight  inches  wide,  and  the  fourth  of  an 
inch  thick,  pierced  with  holes  a  quarter  of  an  inch  apart,  as 
shown  by  Fig.  41.  This  being  placed  in  a  horizontal  position, 
and  shaken,  the  caps  find  their  way  into  the  apertures,  with 
their  open  ends  up,  in  a  manner  that  is  quite  surprising. 

FIG.  41. 


Mode  of  Placing  the    Caps  for  Fitting. 

266.  Third. — A  piece  of  brass  plate,  of  the  exact  size  of  that 
containing  the  caps,  is  pierced  with  apertures  to  correspond  with 
each  and  every  cap,  but  smaller  in  size,  as  shown  by  Fig.  42. 


FIG.  42. 





Mode  of  Filling. 


This  plate,  being  about  the  sixth  of  an  inch  thick,  is  laid  on 
a  smooth  surface,  and  the  fulminating  compound,  a  little  moist- 
ened by  gum-water,  is  rubbed  into  the  apertures  with  the  hand, 
where  it  adheres,  that  remaining  on  the  surface  being  rubbed  off. 


RESULTANT    MOTION.  63 

267.  Fourth. — This  brass  plate,  being  laid  on  that  Contain- 
ing the  caps,  each  aperture  corresponding  to  a  cap,  the  powder, 
by  means  of  a  brush,  is  made  to  fall  into  the  caps. 

^268.  Fifth. — The  caps  are  now  charged  with  the  powder, 
in  a  loose  state,  and  requires  a  gentle  pressure  to  fix  it  in  its 
place. 

This  is  done  by  placing  the  plate,  containing  them,  as  above 
described,  under  rows  of  punches,  which  are  worked  with  a  little 
cam  engine,  and  by  which  the  punches  are  lifted,  while  the 
plate  is  moved  forward,  by  means  of  a  click  and  notches,  so  as 
to  correspond  exactly  with  the  fall  of  the  punches,  by  the  press- 
ure of  which,  the  powder  is  fixed  in  its  place. 

269.  Sixth. — The  best  caps  are  varnished,  in  order  to  pro- 
tect them  from   moisture.     It  being  the  powder  only  which 
requires  this  protection,  in  France  it  is  done  with  a  little  brush 
on  each  cap  held  in  the  fingers.     But  Mr.  Mclntyre  has  invented 
a  much  more  expeditious  way,  and  which  insures  the  same 
quantity  in  each  cap. 

This  is  done  by  a  small  machine,  consisting  of  two  cams  ;  a 
click  working  in  horizontal  notches,  and  a  crank,  by  which 
the  whole  is  moved.  On  the  platform  or  bed  of  this,  is  laid  the 
plate,  Fig.  41,  containing  the  caps,  (268,)  and  on  working  the 
machine,  two  dozen  blunt  metallic  points  are  alternately  dipped 
into  a  little  trough  containing  copal  varnish,  and  then  into  the 
caps,  these  being  moved  by  the  click,  to  correspond  with  the 
revolution  of  the  cams  by  which  the  motions  of  these  points 
are  produced.  In  this  way  hundreds  of  caps  are  varnished  in 
a  few  minutes. 

270.  Seventh. — The  edges  of  the  best  caps  are  polished,  one 
at  a  time,  by  holding  them  with  pliers  for  a  second  on  a  spindle 
of  steel,  revolving  a  thousand  times   a  minute,  the  point  of 
which  enters  the  cap,  the  edge  rubbing  against  a  shoulder,  by 
which  the  work  is  done. 

With  two  engines,  as  above  described,  the  proprietor  esti- 
mates the  number  of  caps  made  per  day,  to  be  about  100,000, 
a  market  being  always  ready  for  all  he  can  make. 

RESULTANT    MOTION. 

271.  Resultant  motion  consists  in  the  operation  of  two,  or 
inore,  forces,  the  joint  action  of  which,  results  in  unity  of  effect. 

271.  What  is  meant  by  resultant  motion  ?  Suppose  two  boats  sailing  at  the  same 
rate  and  in  the  same  direction,  if  an  apple  be  tossed  from  one  to  the  other,  what  wUJ 
be  its  direction  in  respect  to  the  boats  ? 


64 


RESULTANT   MOTION. 


FIG.  43. 


Suppose  two  men  to  be  sailing  in  two  boats,  each  at  the 
rate  of  four  miles  an  hour,  at  a  short  distance  opposite  to  each 
other,  and  suppose  as  they  are  sailing  along  in  this  manner,  one 
of  the  men  throws  the  other  an  apple.  In  respect  to  the  boats, 
the  apple  would  pass  directly  across  from  one  to  the  other,  that 
is,  its  line  of  direction  would  be  at  right-angles  with  the  sides 
of  the  boats.  But  its  actual  line  through  the  air  would  be 
oblique,  or  diagonal,  in  respect  to  the  sides  of  the  boats,  because, 
in  passing  from  boat  to  boat,  it  is  impelled  by  two  forces,  viz., 
the  force  of  the  motion  of  the  boat  forward,  and  the  force  by 
which  it  is  thrown  by  the  hand  across  this  motion. 

272.  This  diagonal  motion 
of  the  apple  is  called  the  re-, 
sultant,  or  the  resulting  mo-      ^ 
tion,  because  it  is  the  effect  or 
result  of  two  motions  resolved 
into  one.     Perhaps  this  will 

be   more  clear   by  Fig.   43, 

where  A  B,  and  C  D,  are  sup- 

posed  to  be  the  sides  of  the  Diagonal  Motion. 

two  boats,  and  the  line  E  F, 

that  of  the  apple.     Now  the  apple,  when  thrown,  has  a  motion 

with  the  boat  at  the  rate  of  four  miles  an  hour,  from  C  toward 

D,  and  this  motion  is  supposed  to  continue  just  as  though  it  had 

remained  in  the  boat. 

273.  Had  it  remained  in  the  boat  during  the  time  it  was 
passing  from  E  to  F,  it  would  have  passed  from  E  to  H.     But  we 
suppose  it  to  have  been  thrown  at  the  rate  of  eight  miles  an 
hour,  in  the  direction  toward  G  ;  and  if  the  boats   are  moving 
south,  and  the  apple  thrown  toward  the  east,  it  would  pass  in 
the  same  time  twice  as  far  toward  the  east  as  it  did  toward  the 
south.     Therefore,  in  respect  to  the  boats  the  apple  would  pass 
at  right-angles  from  the  side  of  one  to  that  of  the  other,  because 
they  are  both  in  motion.     But  in  respect  to  a  right  line,  drawn 
from  the  point  where  the  apple -was  thrown,  and  a  parallel  line 
with  this,  drawn  from  the  point  where  it  strikes  the  other  boat, 
the  line  of  the  apple  would  be  oblique.     This  will  be  clear,  when 
we  consider  that,  when  the  apple  is  thrown,  the  boats  are  at  the 
points  E  and  G,  and  that  when  it  strikes,  they  are  at  II  and  F, 
these  two  points  being  opposite  to  each  other. 

What  would  be  its  line  through  the  air  in  respect  to  the  boats?  272.  What  is  this 
kind  of  motion  called  f  Why  is  it  called  resultant  motion  I  Explain  Fig  43  273. 
Why  woul'J  the  line  of  the  apple  he  actually  at  right-angles  in  respect  to  the  boats, 
but  oblique  in  respect  to  parallel  lines  drawn  from  where  it  was  thrown  and  where 
it  struck  1  How  is  this  further  illustrated  1 


HOROLOQT.  65 

The  line  E  F,  through  which  the  apple  is  thrown,  is  called 
the  diagonal  of  a  parallelogram,  as  already  explained  under 
compound  motion. 

274.  On  the  above  principle,  if  two  ships,  during  a. battle,  are 
sailing  before  the  wind  at  equal  rates,  the  aim  of  the  gunners 
will  be  exactly  the  same  as  though  they  stood  still ;  whereas,  if 
the  gunner  fires  from  a  ship  standing  still,  at  another  under  sail, 
he  takes  his  aim  forward-  of  the  mark  he  intends  to  hit,  because 
the  ship  would  pass  a  little  forward  while  the  ball  is  going  to  her. 

275.  And  so,  on  the  contrary,  if  a  ship  in  motion  fires  at  an- 
other standing  still,  the  aim  must  be  behind  the  mark,  because, 
as   the  motion  of  the  ball  partakes  of  that  of  the  ship,  it  will 
strike  forward  at  the  point  aimed  at. 

276.  For  the  same  reason,  if  a  ball  be  dropped  from  the  top- 
mast of  a  ship  under  sail,  it  partakes  of  the  motion  of  the  ship 
forward,  and  will  fall  in  a  line  with  the  mast,  and  strike  the  same 
point  on  the  deck  as  though  the  ship  stood  still. 

If  a  man  upon  the  full  run  drops  a  bullet  before  him  from 
the  height  of  his  head,  he  can  not  run  so  fast  as  to  overtake  it 
before  it  reaches  the  ground. 

It  is  on  this  principle,  that  if  a  cannon-ball  be  shot  up  verti- 
cally from  the  earth,  it  will  fall  back  to  the  same  point ;  for. 
although  the  earth  moves  forward  while  the  ball  is  in  the  air, 
yet,  as  it  carries  this  motion  with  it,  so  the  ball  moves  forward, 
also,  in  an  equal  degree,  and,  therefore,  comes  down  at  the 
same  place. 


HOROLOGY. 


277.  This  term,  derived  from  the  Greek,  means,  to  indicate 
the  hour.     It  is  the  science  of  time-keeping. 

278.  For  this  purpose,  a  great  variety  of  instruments  have 
been  invented,  by  some  of  which,  time  was  measured  by  the 
dropping  of  water, -as  in  the  clepsydra,  or  water-clock,  in  others, 
by  the  running  of  sand,  as  in  the  hour-glass,  or  by  the  revolu- 
tion of  the  sun,  by  means  of  the  gnomon,  or  sun-dial.     But 
these  ancient  methods  have  given  place  to  the  modern  inven- 
tion of  clocks,  regulated  by  the  pendulum,  and  watches,  regu- 
lated bv  a  balance -wheel. 


274.  When  the  ships  are  in  equal  motion,  where  does  the  gunner  take  his  aim? 
Why  does  he  aim  forward  of  the  mark  when  the  other  ship  is  in  motion  ?  275  If  a 
eh  p-'m  motion  fires  af  one  Handing  still,  where  must  be  the  aim  ?  Why.  in  this  case, 
imiM  the  aim  be  brhinri  the  mark  ?  276.  What  other  illustrations  are  given  of  result- 
ant motion  1  277.  What  is  the  meaning  of  horology  1  278.  What  were  the  ancient 
methods  of  keeping  time  ? 


PENDULUM. 


PENDULUM. 


FIG,  44. 


Pendulum. 


2*79.  A  pendulum  is  a  heavy  body,  suck  as  a  piece  of  brass 
or  lead,  suspended  by  a  wire  or  cord,  so  as  to  swing  backward 
and  forward. 

When  a  pendulum  swings,  it  is  said  to  vibrate  ;  and  that 
part  of  a  circle  through  which  it  vibrates,  is  called  its  arc. 

280.  The  times  of  the  vibration  of  a  pendulum  are  very 
nearly  equal,  whether  it  pass  through  a  greater  or  less  part  of 
its  arc. 

Suppose  A  and 
B,  Fig.  44,  to  be 
two  pendulums  of 
equal  length,  and 
suppose  the  weights 
of  each  are  carried, 
the  one  to  C,  and 
the  other  to  D,  and 
both  let  fall  at  the 
same  instant  ;  their 
vibrations  would  be 

equal  in  respect  to  time,  the  one  passing  through  its  arc  from 
C  to  E,  and  so  back  again  in  the  same  time  that  the  other 
passes  from  D  to  F,  and  back  again. 

281.  The  reason  of  this  appears  to  be,  that  when  the  pendu- 
lum is  raised  high,  the  action  of  gravity  draws  it  more  directly 
downward,  and  it  therefore  acquires  in  falling  a  greater  com- 
parative velocity  than  is  proportioned  to  the  trifling  difference 
of  height. 

282.  Common  Clock.  —  In  the  common  clock,  the  pendulum 
is  connected  with  wheel-work,   to  regulate  the  motion  of  the 
hands,  and  with  weights,  by  which  the  whole  is  moved.     The 
vibrations   of  the   pendulum    are   numbered   by  a  wheel   or 
escapement,    having   thirty  teeth,  which    revolves  once  in    a 
minute.     Each  tooth,  therefore,  answers  to  one  vibration  of  the 
pendulum,  and  the  wheel  moves  forward  one  tooth  in  a  second, 
Thus  the  second-hand  revolves  once  in  every  sixty  beats  of  the 
pendulum  ;  and,  as  these  beats  are  seconds,  it  goes  round  once 
in  a  minute.     By  the  pendulum  the  whole  machine  is  regu- 
lated, for  the  clock  goes  faster  or  slower,  according  to  its  num- 

279.  What  is  a  pendulum  1  280.  What  is  meant  by  the  vibration  of  a  pendulum  1 
What  is  that  part  of  a  circle  called  through  which  it  swings?  281.  Why  doe?  the 
pendulum  vibrate  in  equal  time  whether  it  goes  through  a  small  or  large  part  of  its 
arc  {  282.  Describe  the  common  clock.  How  many  vibrations  has  the  pendulum  in 
a  minute  1 


PENDULUM. 


ber  of  vibrations  in  a  given  time.  The  number  of  vibrations 
which  a  pendulum  makes  in  a  given  time  depends  upon  its 
length,  because  a  long  pendulum  does  not  perform  its  jour- 
ney to  and  from  the  corresponding  points  of  its  arc  so  soon  as  a 
short  one. 

283.  As  the  motion  of  the  clock  is  regulated  entirely  by  the 
pendulum,  and  as  the  number  of  vibrations  are  as  its  length, 
the  least  variation  in  this  respect  will   alter  its  rate  of  going. 
To  beat  seconds,  its  length  must  be  about  thirty-nine  inches. 
In  the  common  clock,  the  length  is  regulated  by  a  screw,  which 
raises  and  lowers  the  weight.     But  as  the  rod  to  which  the 
weight  is  attached  is  subject  to  variations  of  length,  in  conse- 
quence of  the  change  of  the  seasons,  being  contracted  by  cold 
and  lengthened  by  heat,  the  common  clock  goes  faster  in  win- 
ter than  in  summer. 

In  the  small  clocks  of  the  present  day, 
the  pendulum  oscillates  twice  and  some- 
times more  in  a  second,  and  consequently 
the  escapement  must  have  60  or  more 
teeth,  the  second-hand  performing  two 
revolutions  in  a  minute. 

The  length  of  a  pendulum  beating  two 
seconds  is  the  square  of  that  beating 
seconds.  If  the  length  of  the  seconds 
pendulum  be  39-}-  inches,  then  that  beat- 
ing two  seconds  will  be  about  13  feet. 

A  pendulum  beating  half  seconds  is  in 
length,  as  the  square  root  of  that  beating 
seconds,  or  about  10  inches  long. 

284.  Gridiron    Pendulum. — Various 
means  have  been  contrived  to  counteract 
the  effects  of  these  changes,   so  that  the 
pendulum  may  continue  the  same  length 
the  whole  year.     Among  inventions  for 
this  purpose,  the  gridiron  pendulum  is 
considered  among  the  best.    It  is  so  called, 
because  it  consists  of  several  rods  of  dif- 
ferent metals  connected  together  at  each 
end. 


Gridiron  Pendulum. 


233.  On  what  depends  the  number  of  vibrations  which  a  pendulum  makes  in  a 
given  time  ?  What  is  the  medium  length  of  the  pendulum  beating  seconds  1  Whj 
does  a  common  clock  go  faster  in  winter  than  in  summer?  What  is  necessary  in 
respect  to  the  pendulum,  to  make  the  clock  go  true  the  year  round  1  281.  What  if 
the  principle  on  which  the  gridiron  pendulum  is  constructed  1 


68  PENDULUM. 

285.  The  principle  on  which  this  pendulum  is  constructed  is 
derived  from  the  fact  that  some  metals  dilate  more  by  the  same 
degrees  of  heat  than  others.     Thus,  brass  will  dilate  about  twice 
as  much  by  heat,  and,  consequently,  contract  twice  as  much  by 
cold,  as  steel.     If,  then,  these   differences  could  be   made  to 
counteract  each  other  mutually,  given  points  at  each  end  of  a 
system  of  such  rods  would  remain  stationary  the  year  round, 
and  thus  the  clock  would  go  at  the  same  rate  in  all  climates,  and 
during  all  seasons. 

286.  Suppose,  then,  steel  bars  A  B,  are  firmly  fixed  to  cross 
bars  at  each  end,  as  seen  by  Fig.  45,  and  that  on  the  lower 
cross  bar,  the  brass  rods  1  2,  are  also  fixed,  then  the  steel  bars 
can  expand  only  downward,  and  the  brass  ones,  only  upward. 
Now  as  the  pendulum  rod  passes  through  the  lower  cross  bar, 
and  is  fixed  to  the  upper  cross  piece  of  the  brass  rods,  it  will  be 
seen  that  the  elongation  of  the  two  metals  by  heat  mutually 
counteract  each  other,  and  therefore  that  the  point  of  suspen- 
sion, a,  and  the  pendulum  weight,  6,  will  always  remain  at  the 
same  'distance  from  each  other.     It  is  found  by  experiment  that 
the  expansion  of  brass  to  that  of  steel  is  in  the  proportion  of 
100  to  61. 

287.  Gravity  varies   the  Vibrations. — As  it  is  the  force  of 
gravity  which   draws  the  weight  of  the  pendulum  from  the 
£ighestx  point  of  its  arc  downward,  and  as  this  force  increases  or 
diminishes  as  bodies  approach  toward  the  center  of  the  earth, 
or  recede  from  it,  so  the  pendulum  will  vibrate  faster  or  slower 
in  proportion  as  this  attraction  is  stronger  or  weaker. 

288.  Now  it  is  known  that  the  earth  at  the  equator  rises 
higher  from  its  center  than  it  does  at  the  poles,  for  toward  the 
poles   it  is  flattened.     The  pendulum,  therefore,   being  more 
strongly  attracted  at  the  poles  than  at  the  equator,  vibrates 
more  rapidly.     For  this  reason,  a  clock  that  would  keep  exact 
time  at  the  equator  would  gain  time  at  the  poles,  for  the  rate  at 
which  a  clock  goes  depends  on  the  number  of  vibrations  its  pen- 
dulum makes.     Therefore,  pendulums,  in  order  to  beat  second;*, 
must  be  shorter  at  the  equator,  and  longer  at  the  poles. 

For  the  same  reason,  a  clock  which  keeps  exact  time  at  the 
foot  of  a  high  mountain,  would  move  slower  on  its  top. 

2$5.  What  are  the  metals  of  which  this  instrument  is  made  ?  286.  Explain  Fig.  45,  and 
give  the  reason  why  the  length  of  the  pendulum  will  not  change  by  the  variations  ot 
temperature.  287.  What  is  the  downward  force  which  makes  the  pendulum  vibrate? 
Explain  the  reason  why  the  same  clock  would  go  faster  at  the  poles  and  slower  at 
the  equator.  288.  How  can  a  clock  which  goes  true  at  the  equator  be  made  to  go 
true  at  the  poles  1  Will  a  clock  keep  equal  time  at  the  foot  and  on  the  top  of  a  high 
mountain  1  Why  will  it  not  ? 


PENDULUM. 


69 


FIG.  46. 


289.  METRONOME. — There  is  a  short  pendulum,  used  by  mu- 
sicians for  marking  time,  which  may  be  made  to  vibrate  fast  or 
slow,  as  occasion  requires.     This   little  instrument  is  called  a 
metronome,  and  besides  the  pendulum,  consists  of  several  wheels, 
and  a  spiral  spring,  by  which  the  whole  is  moved.     This  pen- 
dulum is  only  ten  or  twelve  inches  long,  and  instead  of  being 
suspended  by  the  end,  like  other  pendulums,  the  rod  is  pro- 
longed above*  the  point  of  suspension,  and  there  is  a  ball  placed 
near  the  upper,  as  well  as  at  the  lower  extremity. 

290.  This  arrangement  will  be  under- 
stood by  Fig.  46,  where  A  is  the  axis 
of  suspension,  B  the  upper  ball,  and  C 
the  lower  one.     Now,  when  this  pendu- 
lum vibrates  from  the  point  A,  the  up- 
per ball  constantly  retards  the  motion  of 
the  lower  one,  by  in  part  counterbalanc- 
ing its  weight,  and  thus  preventing  its 
full  velocity  downward. 

291.  Perhaps   this  will  be  more  ap- 
parent, by  placing  the  pendulum,  Fig. 
47,  for  a  moment  on  its  side,  and  across 
a  bar,  at  the  point  of  suspension.     In 
this  position,    it  will  be  seen  that   the 
little  ball  would  prevent  the  large  one 
from  falling  with  its  full  weight,   since, 
were  it  moved  to  a  cer- 
tain   distance    from    the 

point  of  suspension,  it 
would  balance  the  large 
one  so  that  it  would  not 
descend  at  alt.  It  is  plain, 

therefore,  that  the  comparative  velocity  of  the  large  ball  will  be 
in  proportion  as  the  small  one  is  moved  to  a  greater  or  less  dis- 
tance from  the  point  of  suspension.  The  metronome  is  so  con- 
structed, the  little  ball  being  made  to  move  up  and  down  on 
the  rod  at  pleasure,  that  its  vibrations  are  made  to  beat  the 
time  of  a  quick  or  slow  tune,  as  occasion  requires. 

By  this  arrangement,  the  instrument  is  made  to  vibrate  every 
two  seconds,  or  every  half,  or  quarter  of  a  second,  at  pleasure. 
Metronome  means  time  measurer. 


FIG.  47. 


Metronome. 


2S9  What  is  the  metronome  1  How  does  this  pendulum  differ  from  the  common 
pendulums?  290.  Explain  Fig.  46.  291.  How  does  the  upper  ball  retard  the  motion 
of  the  lower  one  ?  How  is  the  metronome  made  to  go  faster  or  slower,  at  pleasure  ? 


CHAPTER   IV. 

MECHANICS. 

292.  Mechanics  is  a  science  which  investigates  the  laws  and 
effects  of  force  and  motion. 

293.  The  practical  object  of  this  science  is,  to  teach  the  best 
modes  of  overcoming  resistances  by  means  of  mechanical  powers, 
and  to  apply  motion  to  useful  purposes,  by  means  of  machinery. 

294.  A  machine  is  any  instrument  by  which  power,  motion, 
or  velocity,  is  applied  or  regulated. 

295.  A^  machine  maybe  very  simple,  or  exceedingly  com- 
plex.    Thus,  a  pin  is  a  machine  for  fastening  clothes,  and  a 
steam-engine  is  a  machine  for  propelling  mills  and  boats. 

As  machines  are  constructed  for  a  vast  variety  of  purposes, 
their  forms,  powers,  and  kinds  of  movement,  must  depend  on 
their  intended  uses. 

Several  considerations  ought  to  precede  the  actual  construc- 
tion of  a  new  or  untried  machine ;  for  if  it  does  not  answer  the 
purpose  intended,  it  is  commonly  a  total  loss  to  the  builder. 

Many  a  man,  on  attempting  to  apply  an  old  principle  to  a 
new  purpose,  or  to  invent  a  new  machine  for  an  old  purpose, 
has  been  sorely  disappointed,  having  found,  when  too  late,  that 
his  time  and  money  had  been  thrown  away,  for  want  of  proper 
reflection,  or  requisite  knowledge. 

If  a  man,  for  instance,  thinks  of  constructing  a  machine  for 
raising  a  ship,  he  ought  to  take  into  consideration  the  inertia  or 
weight,  to  be  moved — the  force  to  be  applied — the  strength  of 
the  materials,  and  the  space  or  situation  he  has  to  work  in. 
For,  if  the  force  applied,  or  the  strength  of  the  materials  be  in- 
sufficient, his  machine  is  obviously  useless  ;  and  if  the  force  and 
strength  be  ample,  but  the  space  be  wanting,  the  same  result 
must  follow. 

If  he  intends  his  machine  for  twisting  the  fibers  of  flexible 
substances  into  threads,  he  may  find  no  difficulty  in  respect  to 
power,  strength  of  materials,  or  space  to  work  in,  but  if  the 

292.  What  is  mechanics  7  293.  What  is  the  object  of  this  science  ?  294  What  is 
a  machine?  295.  Mention  one  of  the  most  simple,  and  one  of  the  most  complex  of 
machines. 


DEFINITIONS.  71 

velocity,  direction,  and  kind  of  motion  he  obtains,  be  not  appli- 
cable to  the  work  intended,  he  still  loses  his  labor. 

Thousands  of  machines  have  been  constructed,  which,  so  far 
as  regarded  the  skill  of  the  workmen,  the  ingenuity  of  the  con- 
triver, and  the  construction  of  the  individual  parts,  were  models 
of  art  and  beauty ;  and,  so  far  as  could  be  seen  without  trial, 
admirably  adapted  to  the  intended  purpose.  But  on  putting 
them  to  actual  use,  it  has  too  often  been  found,  that  their  only 
imperfection  consisted  in  a  stubborn  refusal  to  do  any  part  of 
the  work  intended. 

Now,  a  thorough  knowledge  of  the  laws  of  motion,  and  the 
principles  of  mechanics,  would,  in  many  instances,,  at  least,  have 
prevented  all  this  loss  of  labor  and  money,  and  spared  him  so 
much  vexation  and  chagrin,  by  showing  the  projector  that  his 
machine  would  not  answer  the  intended  purpose. 

The  importance  of  this  kind  of  knowledge  is  therefore  ob- 
vious, and  it  is  hoped  will  become  more  so  as  we  proceed. 

DEFINITIONS. 

296.  In  mechanics,  as  well  as  in  other  sciences,  there  are 
words  which  must  be  explained,  either  because  they  are  com- 
mon words  used  in  a  peculiar  sense,  or  because  they  are  terms 
of  art,  not  in  common  use.     All  technical  terms  will  be  as  much 
as  possible  avoided,  but  still  there  are  a  few,  which  it  is  neces- 
sary here  to  explain. 

297.  Force  is  the  means  by  which  bodies  are  set  in  motion, 
kept  in  motion,  and  when  moving,  are  brought  to  rest. 
The  force  of  gunpowder  sets  the  ball  in  motion,  and  keeps 
it  moving,  until  the  force  of  the  resisting  air,  and  the  force 
of  gravity,  bring  it  to  rest. 

298.  Power  is  the  means  by  which  the  machine  is  moved, 
and  the  force  gained.     Thus  we  have  horse-power,  water- 
power,  and  the  power  of  weights. 

299.  Weight  is  the  resistance,  or  the  thing  to  be  moved  by 
the  force  of  the  power.     Thus  the  stone  is  the  weight  to 
be  moved  by  the  force  of  the  lever  or  bar. 

300.  Fulcrum,  or  prop,  is  the  point  on  which  a  thing  is  sup- 
ported, and  about  which  it  has  more  or  less  motion.     In 
raising  a  stone,  the  thing  on  which  the  lever  rests,  is  the 
fulcrum. 


297.  What  is  meant  by  force  in  mechanics  ?  295.  What  is  meant  by  power  ?  299. 
What  is  understood  by  weight  1  300.  What  is  the  fulcrum  1  301.  Are  the  mechan- 
ical powers  numerous,  or  only  few  in  number  7 


72  LEVER. 

301.  In  mechanics,  there  are  a  few  simple  machines  called 
the  mechanical  powers,  and  however  mixed,  or  complex,  a  com- 
bination of  machinery  may  be,  it  consists  only  of  these  few  in- 
dividual powers. 

We  shall  not  here  burden,  the  memory  of  the  pupil  with  the 
names  of  these  powers,  of  the  nature  of  which  he  is  at  present 
supposed  to  know  nothing,  but  shall  explain  the  action  and  use 
of  each  in  its  turn,  and  then  sum  up  the  whole  for  his  accom- 
modation. 


THE    LEVER. 


302.  Any  rod,  or  bar,  which  is  used  in  raising  a  weight,  or 
surmounting  a  resistance,  by  being  placed  on  a  fulcrum,  or  prop, 
becomes  a  lever.     Levers  are  simple  and  compound. 

303.  Simple  levers  are  of  three  kinds,  namely :  first,  where 
the  fulcrum  is  between  the  power  and  the  weight;  second, 
where  the  weight  is  between  the  fulcrum  and  the  power  ;  third, 
where  the  power  is  between  the  fulcrum  and  the  weight* 

304.  First  Kind. — The  first  kind  is  represented  by  Fig.  48, 
being    a   straight 

rod  of  iron,  called  FIG  48- 

a  crowbar,  in  com- 
mon use  for  rais- 
ing rocks  and  oth- 
er heavy  bodies. 
The  stone,  B,  is 
the  weight.  A  the 

7  -j-xs.il  Simple  Lever. 

lever,  and  C   the 

fulcrum  j  the  power  being  the  hand  of  a  man  applied  at  A. 

It  will  be  observed,  that  by  this  arrangement  the  application 
of  a  small  power  may  be  used  to  overcome  a  great  resistance. 

305.  The  force  to  be  obtained  by  the  lever,  depends  on  its 
length,  together  with  the  power  applied,  and  the  distance  of  the 
weight  and  power  from  the  fulcrum. 

306.  Suppose,  Fig.  49,  that  A  is  the  lever,  B  the  fulcrum,  D 
the  weight  to  be  raised,  and  C  the  power.     Let  D  be  considered 
three  times  as  heavy  as  C,  and  the  fulcrum  three  times  as  far 
from  C  as  it  is  from  D ;  then  the  weight  and  power  will  ex- 
actly balance  each  other.     Thus,  if  the  bar  be  four  feet  longv 

302.  What  is  a  lever  ?  303.  What  are  the  three  kinds  of  simple  levers  ?  304.  What 
is  the  simplest  of  all  mechanical  powers  \  Explain  Fig.  48.  Which  is  the  weight  1 
Where  is  the  fulcrum  ?  Where  is  the  power  applied  !  What  is  the  power  in  this 
ease?  305.  On  what  does  the  force  to  be  obtained  by  the  lever  depend  }  306  Sup- 
pose a  lever  four  feet  long,  and  the  fulcrum  one  foot  from  the  end,  what  number  of 
pound* will  balance  each  other  at  the  ends  1 


LEVER.  73 

FIG.  49. 


Lever —  Unequal  Arms. 

and  the  fulcrum  three  feet  from  the  end,  then  three  pounds  on 
the  long  arm  will  weigh  just  as  much  as  nine  pounds  on  the 
short  arm,  and  these  proportions  will  be  found  the  same  in  all 
cases. 

307.  When  two  weights  balance  each  other,  the  fulcrum  is 
always  at  the  center  of  gravity  between  them,  and  therefore, 
to  make  a  small  weight  raise  a  large  one,  the  fulcrum  must  be 
placed  as  near  as  possible  to  the  large  one,  since  the  greater  the 
distance  from  the  fulcrum  the  small  weight  or  power  is  placed, 
the  greater  will  be  its  force. 

FIG.  50. 


I 

Q~ 


Lever— Double  Weights. 


308.  Suppose  the  weight  B,  Fig.  50,  to  be  sixteen  pounds, 
and  suppose  the  fulcrum  to  be  placed  so  near  it,  as  to  be  raised 
by  the  power  A,  of  four  pounds  hanging  equally  distant  from 
the  fulcrum  and  the  end  of  the  lever.     If  now  tlie  power  A  be 
removed,  and  another  of  two  pounds,  C,  be  placed  at  the  end 
of  the  lever,  its  force  will  be  just  equal   to  A,  placed  at  the 
middle  of  the  lever. 

309.  But  let  the  fulcrum  be  moved  along  to  the  middle  of 
the  lever,  with  the  weight  of  sixteen  pounds  still  suspended  to 
it,  it  would  then  take  another  weight  of  sixteen  pounds,  instead 
cf  two  pounds,  to  balance  it,  Fig.  51. 

307.  When  weights  ba'ance  each  other,  at  what  point  between  them  must  the  ful- 
crum be  ?  38  Suppose  a  weight  of  16  pounds  on  the  short  arm  of  a  lever  is  coun- 
terbalanced by  4  pounds  in  the  middle  of  the  long  arm.  what  power  would  balance 
this  weight  at  the  end  of  the  lever  ?  309.  Suppose  the  fulcrum  to  be  moved  to  the 
middle  of  the  lever,  what  power  would  then  be  equal  to  16  pounds? 

4 


74  LEVER. 

FIG.  61. 


Lever — Equal  Arms. 

Thus,  the  power  which  would  balance  sixteen  pounds,  when 
the  fulcrum  is  in  one  place,  must  be  exchanged  for  another  power 
weighing  eight  times  as  much,  when  the  fulcrum  is  in  another 
place. 

310.  From  these  investigations,  we  may  draw  the  following 
general  truth,  or  proposition,  concerning  the  lever :  "  That  the 
force  of  the  lever  increases  in  proportion  to  the  distance  of  the 
power  from  the  fulcrum,  and  diminishes  in  proportion  as  the 
distance  of  the  weight  from  the  fulcrum  increases." 

311.  From  this  proposition,  may  be  drawn  the  following  rule, 
by  which  the  exact  proportions  between  the  weight  or  resist- 
ance, and  the  power,  may  be  found.     Multiply  the  weight  by 
its  distance  from  the  fulcrum  ;   then  multiply  the  power  by  its 
distance  from  the  same  point,  and  if  the  products  are  equal,  the 
weight  and  the  power  will  balance  each  other. 

312.  Suppose  a  weight  of  100  pounds  on  the  short  arm  of 
a  lever,  8  inches  from  the  fulcrum,  then  another  weight,  or 
power,  of  8  pounds,  would  be  equal  to  this,  at  the  distance 
of  100  inches  fro  n  the  fulcrum ;  because  8  multiplied  by  100 
is  equal  to  800 ;  and  100  multiplied  by  8  is  equal  to  800,  and 
thus  they  would  mutually  counteract  each  other. 

313.  Many  instruments 

in  common  use  are  on  the  FIG.  52. 

principle  of  this  kind   of 

lever.     Scissors,  Fig.  52, 

consist  of  two  levers,  the 

rivet  being   the  fulcrum 

for  both.     The  fingers  are 

the  power,  and  the  cloth 

to  be  cut,  the  resistance  to 

be  overcome.  TWO  Levers. 

Pincers,    forceps,    and 
sugar-cutters,  are  examples  of  this  kind  of  lever. 

310.  What  is  the  general  proposition  drawn  from  these  results?  311.  What  is  the 
rule  for  finding  the  proportions  between  the  weight  and  power  1  312.  Give  an  illus- 
tration of  this  rulp.  313.  What  instruments  operate  on  the  principle  of  this  lever  7 


LEVIR.  75 

314.  A  common  scale-beam,  used  for  weighing,  is  a  lever, 
suspended  at  the  center  of  gravity,  so  that  the  two  arms  bal- 
ance each  other.     Hence  the  machine  is  called  a  balance.     The 
fulcrum,  or  what  is  called  the  pivot,  is  sharpened,  like  a  wedge, 
and  made  of  hardened  steel,  so  as  much  as  possible  to.  avoid 
friction. 

315.  A  dish  is  suspended  by  FIG.  53. 
cords  to  each  end  or  arm  of  the 

lever,  for  the  purpose  of  hold- 
ing the  articles  to  be  weighed. 
"When  the  whole  is  suspended 
at  the  point  a,  Fig.  53,  the 
beam  or  lever  ought  to  remain 
in  a  horizontal  position,  one  of  common  Scale*. 

its  ends  being  exactly  as  high 

as  the  other.  If  the  weights  in  the  two  dishes  are  equal,  and 
the  support  exactly  in  the  center,  they  will  always  hang  as 
represented  in  the  figure. 

316.  A  very  slight  variation  of  the  point  of  support  toward 
one  end  of  the  lever,  will  make  a  difference  in  the  weights  em- 
ployed to  balance  each  other.     In  weighing  a  pound  of  sugar, 
with  a  scale-beam  of  eight  inches  long,  if  the  point  of  support 
is  half  an  inch  too  near  the  weight,  the  buyer  would  be  cheated 
nearly  one  ounce,  and  consequently  nearly  one  pound  in  every 
sixteen  pounds.     This  fraud  might  instantly  be  detected  by 
changing  the  places  of  the  sugar  and  weight,  for  then  the  dif- 
ference would  be  quite  material,  since  the  sugar  would  then 
seem  to  want  twice  as  much  additional  weight  as  it  did  really 
want. 

317.  The  steelyard  dif-  ^ 
fers  from  the  balance,  in 

having  its  support  near 
one  end,  instead  of  in  the 
middle,  and  also  in  hav- 
ing the  weights  suspend- 
ed by  hooks,  instead  of  steelyard. 
being  placed  in  a  dish. 

If  we  suppose  the  beam  to  be  7  inches  long,  and  the  hook, 
C,  Fig.  54,  to  be  one  inch  from  the  end,  then  the  pound  weight, 
A,  will  require  an  additional  pound  at  B,  for  every  inch  it  is 

314  In  the  common  scale-beam,  where  is  the  fulcrum  ?  315.  In  what  position 
ought  the  scale-beam  to  hang  ]  316.  How  may  a  fraudulent  scale-beam  be  made  7 
How  may  the  cheat  be  detected '}  317.  How  does  the  steelyard  differ  Irom  the 
balance  ? 


76  LEVER. 

moved  from  it.     This,  however,  supposes  that  the  bar  will  bal- 
ance itself,  before  any  weights  are  attached  to  it. 


FIG.  55. 


Lever  of  the  Second  Kind. 

318.  Second  Kind. — The  second  kind  of  lever  is  represented 
by  Fig.  55,  where  W  is  the  weight,  L  the  lever,  F  the  fulcrum, 
and  P  a  pulley,  over  which  a  string  is  thrown,  and  a  weight 
suspended,  as  the  power.     In  the  common  use  of  a  lever  of  the 
first  kind,  the  force  is  gained  by  bearing  down  the  long  arm, 
which  is  called  prying.     In  the  second  kind,  the  force  is  gained 
by  carrying  the  long  arm  in  a  contrary  direction,  or  upward, 
and  this  is  called  lifting. 

319.  Levers  of  the  second  kind  are  not  so  common  as  the 
first,  but  are  frequently  used  for  certain  purposes.     The  oars  of 
a  boat  are  examples  of  the  second  kind.     The  water  against 
which  the  blade  of  the  oar  pushes,  is  the  fulcrum,  the  boat  is 
the  weight  to  be  moved,  and  the  hands  of  the  man,  the  power. 

320.  Two  men  carrying  a  load  between  them  on  a  pole,  is 
also  an  example  of  this  kind  of  lever.     Each  man  acts  as  the 
power  in  moving  the  weight,  and  at  the  same  time  each  be- 
comes the  fulcrum  in  respect  to  the  other. 

If  the  weight  happens  to  slide  on  the  pole,  the  man  toward 
whom  it  goes  has  to  bear  more  of  it  in  proportion  as  its  dis- 
tance from  him  is  less  than  before. 

321.  A  load  at  A,  Fig.  56,  is  borne  equally  by  the  two  men, 
being  equally  distant  from  each  other ;  but  at  B,  three  quarters 
of  its  weight  would  be  on  the  man  at  that  end,  because  three 
quarters  of  the  length  of  the  lever  would  be  on  the  side  of  the 
other  man. 


318.  la  the  first  kind  of  lever,  where  is  the  fulcrum,  in  respect  to  the  weight  and 
power  1  In  the  second  kind,  where  is  the  fulcrum,  in  respect  to  the  weight  and 
power  1  What  is  the  action  of  the  first  kind  called  ?  What  is  the  action  of  the  see* 
ond  kind  called  1  319.  Give  examples  of  the  second  kind  of  lever.  320.  In  rowing  a 
boat,  what  is  the  fulcrum,  what  the  weight,  and  what  the  power  ?  321.  What  other 
illustrations  ef  this  principle  ar«  given  1 


LEVER. 


FIG.  56. 
.A 


Lever — Unequal  Arm*. 


Lever  of  the  Third  Kind. 


I 


322.  Third  Kind. — In  the  third  and  last  kind  of  lever,  the 
weight  is  placed  at  one  end,  the  fulcrum  at  the  other  end,  and 
the  power  between  them,  or  the  hand,  is  between  the  fulcrum 
and  the  weight. 

This  is  represented  by  Fig.  57,  where  C  is  the  fulcrum,  A 
the  power,  suspended  over  the  pulley  B,  and  D  is  the  weight 
to  be  raised. 

323.  This  kind  of  lever  works  to  great  disadvantage,  since 
the  power  must  be  greater  than  the  weight.     It  is  therefore 
seldom  used,  except  in  cases  where  velocity  and  not  force  is 
required.     In  raising  a  ladder  from  the  ground  to  the  roof  of 
a  house,  men  are  obliged  sometimes  to  make  use  of  this  princi- 
ple, and  the  great  difficulty  of  doing  so,  illustrates  the  mechan- 
ical disadvantage  of  this  kind  of  lever. 

We  have  now  described  the  three  kinds  of  levers,  and,  we 
hope,  have  made  the  manner  in  which  each  kind  acts  plain,  by 
illustrations.  But  to  make  the  difference  between  them  still 
more  obvious,  and  to  avoid  all  confusion,  we  will  here  compare 
them  together. 


322  In  the  third  kind  of  lever  where  are  the  respective  places  of  the  weight, 
power,  and  fulcrum  ?  323.  What  is  the  disadvantage  of  this  kind  of  lever  ?  Give  an 
example  of  the  use  of  the  third  kind  of  lever  7 


LEVflR. 
FIG.  58. 


i 


FIG.  69. 


no.  GO. 


The  Levers  Compared. 

324.  In  Fig.  58,  the  weight  and  hand  both  act  downward. 
In  59,  the  weight  and  hand  act  in  contrary  directions,  the  hand 
upward  and  the  weight  downward,  the  weight  being  between 
them.     In  60,  the  hand  and  weight  also  act  in  contrary  direc- 
tions, but  the  hand  is  between  the  fulcrum  and  the  weight. 

325.  COMPOUND   LEVER. — When  several   simple   levers   ar<? 
connected  together,  and  act  one  upon  the  other,  the  machine  is 


324.  In  what  direction  dp  the  hand  and  weight  act,  in  the  first  kind  of  lever?  In 
what  direction  do  they  act  in  the  second  kind  ]  In  what  direction  do  they  act  in  the 
third  kind  1  325.  What  Is  a  compound  lever  7 


LEVER.  79 

called  a  compound  lever.  In  this  machine,  as  each  lever  acts  as 
an  individual,  and  with  a  force  equal  to  the  action  of  the  next 
lever  upon  it,  the  force  is  increased  or  diminished,  and  becomes 
greater  or  less,  in  proportion  to  the  number  or  kind  of  levers 
employed. 

We  will  illustrate  this  kind  of  lever  by  a  single  example,  but 
must  refer  the  inquisitive  student  to  more  extended  works  for  a 
full  investigation  of  the  subject. ' 

FIG.  61. 
E       T    P 


Compound  Levtr. 

Fig.  61  represents  a  compound  lever,  consisting  of  three  sim- 
ple levers  of  the  first  kind. 

326.  In  calculating  the  force  of  this  lever,  the  rule  applies 
which  has  already  been  given  for  the  simple  lever,  namely : 
The  length  of  the  long  arm  is  to  be  multiplied  by  the  moving 
power,  and  that  of  the  short  one,  by  the  weight,  or  resistance. 

327.  Let  us  suppose,  then,  that  the  three  levers  in  the  figure 
Sire  of  the  same  length,  the  long  arms  being  six  inches,  and  the 
short  ones  two  inches  long ;  required,  the  weight  which  a  mov- 
ing power  of  1  pound  at  A  will  balance  at  B.     In  the  first  place, 
1  pound  at  A,  would  balance  3  pounds  at  E,  for  the  lever  being 
6  inches,  and  the  power  1  pound,  6x1  =  6,  and  the  short  one 
being  2  inches,  2x3  =  6.     The  long  arm  of  the  second  lever 
being  also  6  inches,  and  moved  with  a  power  of  3  pounds,  mul- 
tiply the  3  by  6  =  18;  and  multiply  the  length  of  the  short 
arm,  being  2   inches,  by  9  =  18.     These  two  products  being 
equal,  the  power  upon  the  long  arm  of  the  third  lever,  at  D, 
would  be  9  pounds.     9  pounds  x6=54,  and  27  X  2,  is  54  ;  so 
that  1  pound  at  A  would  balance  27  at  B. 

The  increase  of  force  is  thus  slow,  because  the  proportion  be- 
tween the  long  and  short  arms  is  only  as  2  to  6,  or  in  the  pro- 
portions of  1,  3,  9. 

326.  By  what  rule  is  the  force  of  the  compound  lever  calculated  ?  327.  How  many 
pounds  weight  will  be  raised  by  three  levers  connected,  of  six  inches  each,  with  the 
fulcrum  two  inches  from  the  end,  by  a  power  of  one  pound  1 


80 


WHEEL    AND    AXLE. 


FIG.  62. 


328.  Now  suppose  the  long  arms  of  these  levers  to  be  18 
inches,  and  the  short  ones  1  inch,  and  the  result  will  be  sur- 
prisingly different,  for  then  1   pound  at  A  would  balance   18 
pounds  at  E,  and  the  second  lever  would  have  a  power  of 
18  pounds.     This  being  multiplied  by  the  length  of  the  lever, 
18x18  —  324  pounds  at  D.     The  third  lever  would  thus  be 
moved  by  a  power  of  324  pounds,  which,  multiplied  by  18 
inches  for  the  weight  it  would  raise,  would  give  5832  pounds. 

329.  The  compound  lever  is  employed  in  the  construction  of 
weighing  machines,  and  particularly  in  cases  where  great  weights 
are  to  be  determined,  in  situations  where  other  machines  would 
be  inconvenient,  on  account  of  their  occupying  too  much  space. 

330.  KNEE  LEVER. — A    compound    instrument,    called    the 
Knee  Lever,  is  used  in  various  kinds  of  machinery,  the  principle 
of  which. is  explained  by  Fig.  62. 

This  combination  consists  of 
a  metal  rod,  A  B,  having  a 
joint  at  A,  above  which  there 
is  a  firm  support.  At  C  is  an- 
other rod,  or  bar,  jointed  to  the 
long  lever,  and  terminating  at 
G,  where  there  is  another  joint, 
attached  to  a  movable  plat- 
form, on  which  the-  force  of  the 
two  levers  are  exerted. 

Now  when  B  is  pushed  to- 
ward the  vertical  position,  the 
force  on  the  joints  A  and  G,  is 
constantly  increased,  until  the 
two  bars  become  perpendicular, 
when  the  pressure  exerted,  is 
augmented  to  nearly  an  indefi-  Knee  Lever. 

nite  degree. 

331.  Various  engines  for  pressing  paper,  and  for  printing,  are 
constructed  on  this  principle,  and  it  is  said  they  are  unequaled 
m  power,  except  by  the  Hydrostatic  press. 


WHEEL    AND    AXLE. 

332.  The  mechanical  power,  next  to  the  lever  in  arrange- 


328.  If  the  long  arms  of  the  levers  be  eighteen  inches,  and  the  short  ones  one  inch 
how  much  will  a  power  of  one  pound  balance  ?  329.  In  what  machines  is  the  com 
pound  lever  employed  1  330.  Explain  the  principle  of  the  Knee  Lever,  Fig  62.  331 
What  machines  are  on  this  principle  1 


WHEEL    AND    AXLE. 


81 


Wheel  and  Axle. 


ment,  is  the  wheel  and  axle.  It  is,  however,  much  more  com- 
plex than  the  lever. 

333.  It  consists  ot  two  wheels,  FIG.  63. 
one  of  which  is  larger   than   the 

other,  but  the  small  one  passes 
through  the  larger,  and  hence  both 
have  a  common  center,  on  which 
they  turn. 

334.  The  manner  in  which  this 
machine  acts  will  be  understood  by 
Fig.  63.     The  large  wheel,  A,  on 
turning  the  machine,  will  take  up, 
or  throw  off,  as  much  more  rope 
than  the  small  wheel  or  axle,  B,  as 

its  circumference  is  greater.  If  we  suppose  the  circumference 
of  the  large  wheel  to  be  four  times  that  of  the  small  one,  then 
it  will  take  up  the  rope  four  times  as  fast.  And  because  A  is 
four  times  as  large  as  B,  1  pound  at  D  will  balance  4  pounds 
at  C,  on  the  opposite  side. 

335.  The  principle  of  this  machine  is 
that  of  the  lever,  as  will  be  apparent 
by  an  examination  of  Fig,  64. 

336.  This  figure  represents  the  ma- 
chine endwise,  so  as  to  show  in  what 
manner  the  lever  operates.     The  two 
weights  hanging  in  opposition  to  each 
other,  the  one  on  the  wheel  at  A,  and 
the  other  on  the  axle  at  B,  act  in  the 
same  manner  as  if  they  were  connected 
by  the  horizontal  lever  A  B,  passing  from 

one  to  the  other,  having  the  common  wheel  and  Axle. 

center,  C,  as  a  fulcrum  between  them. 

337.  The  wheel  and  axle,  therefore,  acts  like  a  constant  suc- 
cession of  levers,  the  long  arm  being  half  the  diameter  of  the 
wheel,  and  the  short  one  half  the  diameter  of  the  axle ;  the 
common  center  of  both  being  the  fulcrum.     The  wheel  and  axle 
has,  therefore,  been  called  the  perpetual  lever. 

338.  The  great  advantage  of  this  mechanical  arrangement  is, 
that  while  a  single  lever  of  the  same  power  can  raise  a  weight 


FIG.  64. 


332.  What  is  the  next  mechanical  power  to  the  lever  ?  333.  Describe  this  ma- 
chine.  334.  Explain  Fig.  63.  335.  On  what  principle  does  this  machine  act  7  336. 
In  Fig  64,  which  is  the  fulcrum,  and  which  the  two  arms  of  the  lever?  337.  What 
is  this  machine  called,  in  reference  to  the  principle  on  which  it  acts?  338  What  is 
the  great  advantage  of  this  machine  over  the  lever  aiid  other  mechanieal  powers 7 

4* 


TTHEEL    AND    AXLK. 


but  a  few  inches  at  a  time,  and  then  only  in  a  certain  direction, 
this  machine  exerts  a  continual  force,  and  in  any  direction 
wanted.  To  change  the  direction,  it  is  only  necessary  that  the 
rope  by  which  the  weight  is  to  be  raised,  should  be  carried  in  a 
line  perpendicular  to  the  axis  of  the  machine,  to  the  place  belovr 
where  the  weight  lies,  and  there  be  let  fall  over  a  pulley. 

339.  Suppose  the  wheel 

and  axle,  Fig.  65,  is  erect-  FIG.  65. 

ed  in  the  third  story  of  a 
store-house,  with  the  axle 
over  the  scuttles,  or  doors 
through  the  floors,  so  that 
goods  can  be  raised  by  it 
from  the  ground-floor,  in 
the  direction  of  the  weight 
A.  Suppose,  also,  that  the 
same  store  stands  on  a 
wharf,  where  ships  come 
up  to  its  side,  and  goods 
are  to  be  removed  from  the 
vessels  into  the  upper  sto- 
ries. Instead  of  removing 
the  goods  into  the  store,  and  hoisting  them  in  the  direction  of 
A,  it  is  only  necessary  to  carry  the  rope  B,  over  the  pulley  C, 
which  is  at  the  end  of  a  strong  beam  projecting  out  from  the 
side  of  the  store,  and  then  the  goods  will  be  raised  in  the  direc- 
tion of  B,  thus  saving  the  labor  of  moving  them  twice. 

The  wheel  and  axle,  under  different  forms,  is  applied  to  a 
variety  of  common  purposes. 

340.  The  capstan,  in  universal  FIG.  66. 
use,  on  board  of  ships,  is  an  axle 

placed  upright,  with  a  head,  or 
drum,  A,  Fig.  66,  pierced  with 
holes  for  the  levers  B,  C,  D.  The 
weight  is  drawn  by  the  rope,  E, 
passing  two  or  three  times  round 
the  axle  to  prevent  its  slipping. 

341.  This  is   a    very  powerful 
and  convenient  machine.     When 
not  in  use,  the  levers  are  taken  out 


Modified  Wheel  and  Axle. 


Capstan. 


339.  Describe  Fig.  65,  and  point  out  the  manner  in  which  weights  can  be  raised  by 
letting  fall  a  rope  over  the  pulley.  340.  What  is  the  capstan  1  Where  is  it  chiefly 
usedl  341.  What  are  the  peculiar  advantages  of  this  form  of  the  wheel  and  axle? 


WHEEL    AND    AXLE. 


83 


Windlass. 


of  their  places  and  laid  aside,  and  when  great  force  is  required, 
two  or  three  men  can  push  at  each  lever. 

342.  WINDLASS.— The 
common  windlass  for 
drawing  water  is  another 
modification  of  the  wheel 
and  axle.  The  winch, 
or  crank,  by  which  it  is 
turned,  is  moved  around 
by  the  hand,  and  there 
is  no  difference  in  the 
principle,  whether  a 
whole  wheel  is  turned, 
or  a  single  spoke.  The 
winch,  therefore,  an- 
swers to  the  wheel,  while  the  rope  is  taken  up,  and  the  weight 
raised  by  the  axle,  as  already  described. 

In  cases  where  great  weights  are  to  be  raised,  and  it  is 
required  that  the  machine  should  be  as  small  as  possible,  on 
account  of  room,  the  simple  wheel  and  axle,  modified  as  repre- 
sented by  Fig.  67,  is  sometimes  used. 

343.  The  axle  may  be  considered  in  two  parts,  one  of  which 
is  larger  than  the  other.     The  rope  is  attached  by  its  two  ends, 
to  the  ends  of  the  axle,  as  seen  in  the  figure.     The  weight  to 
be  raised  is  attached  to  a  small  pulley,  around  which  the  rope 
passes.     The  elevation  of  the  weight  may  be  thus  described. 
Upon  turning  the  axle,  the  rope  is  coiled  around  the  larger  part, 
and,  at  the  same  time,  it  is  thrown  off  the  smaller  part.     At 
every  revolution,  therefore,  a  portion  of  the  rope  will  be  drawn 
up,  equal  to  the  circumference  of  the  thicker  part,  and  at  the 
same  time  a  portion,  equal  to  that  of  the  thinner  part,  will  be 
let  down.     On  the  whole,  then,  one  revolution  of  the  machine 
will  shorten  the  rope  where  the  weight  is  suspended,  just  as 
much  as  the  difference  is  between  the  circumference  of  the  two 
parts. 

344.  Illustration. — Now  to  understand  the  principle  on  which 
this  machine  acts,  we  must  refer  to  Fig.  68,  where  it  is  obvious 
that  the  two  parts  of  the  rope,  A  and  B,  equally  support  the 
weight  D,  and  that  the  rope,  as  the  machine  turns,  passes  from 
the  small  part  of  the  axle  E  to  the  large  part  H,  consequently, 


342.  In  the  common  windlass,  what  part  answers  to  the  wheel  ?  Explain  Fig.  67 
343.  Wliy  is  the  rope  shortened,  and  the  weight  raised  J  344  What  is  the  design  o. 
Fig.  68  ?  Does  the  weight  rise  perpendicular  to  the  axis  of  motion  ? 


84 


WHEEL    AND    AXLE. 


Windlass. 


the  weight  does  not  rise  in  a  perpendicular  FIG.  68. 

line  toward  C,  the  center  of  both,  but  in  a 
line  between  the  outsides  of  the  large  and 
small  parts. 

345.  Let  us  consider  what  would  be  the 
consequence  of  changing  the  rope  A  to  the 
larger  part  of  the  axle,  so  as  to  place  the 
weight  in  a  line  perpendicular  to  the  axis  of 
motion.     In  this  case,  it  is  obvious  that  the 
machine  would  be  in  equilibrium,  since  the 
weight  D  would  be  divided  between  the  two 
sides  equally,  and  the  two  arms  of  the  lever 
passing  through  the  center  C,  would  be  of 
equal  length,  and  therefore  no  advantage 
would  be  gained. 

346.  But  in  the  actual  arrangement,  the  weight  being  sus- 
tained equally  by  the  large  and  small  parts,  there  is  involved  a 
lever  power,  the  long  arm  of  which  is  equal  to  half  the  diameter 
of  the  large  part,  while  the  short  arm  is  equal  to  half  the  diam- 
eter of  the  small  part,  the  fulcrum  being  between  them. 

A  Varying  Power,  producing  a  Constant  Force. — If  a 
power,  varying  under  any  given  conditions,  be  required  to  over- 
come a  resistance  which  varies  according  to  some  other  given 
conditions,  the  one  may  be  accommodated  to  the  other  by  pro- 
ducing a  variation  in  the  leverage,  by  which  one  or  both  acts. 

347.  This  is  done  in  the 

mechanism  of  the  watch,  of,  FIG.  69. 

which  A,  Fig.  69,  is  the  bar- 
rel containing  the  power  in 
the  form  of  a  convoluted 
spring,  and  B  the  fusee  which 
acts  as  a  varying  lever,  and 
through  which  motion  is 
conveyed  to  the  hands  of  the 
watch. 

348.  Now  when  the  watch  is  first  wound  up,  the  main-spring 
within  the  barrel  is  closely  coiled,  and  of  course  acts  with  much 
more  power  than  afterward,  when  it  is  partly  unrolled  ;  hence, 
were  no  means  used  to  equalize  this  power,  every  watch  would 


Barrel  and  Fusee. 


345.  Suppose  the  cylinder  was.  throughout,  of  the  same  size,  what  would  be  the 
consequence?  3-)6.  On  what  principle  does  this  machine  act 1  Which  are  the  long 
and  short  arms  of  the  lever,  and  where  is  the  fulcrum?  347.  What  is  the  main 
spring  of  a  watch  ?  348.  Where  is  it  contained  ?  What  is  the  fusee  of  a  watch  1 
What  is  its  form  1  When  does  the  main  spring  act  with  most  force  1 


WHEEL    AND    AXLE. 


85 


run   two   or   three  times  as   fast,    when   first  wound  up,   as 
afterward. 

349.  We  shall  see  that  the  fusee  is  a  complete  remedy  for 
the  varying  action  of  the  main-spring.     Its  form  is  a  low  cone, 
with  its  surface  cut  into  a  spiral  groove,  to  receive  the  chain, 
which  runs  round  the  barrel.     Now  when  the  watch  is  wound 
up,  by  applying  the  key  to  the  axis  of  the  fusee  at  C,  the  main- 
spring, one  end  of  which  is  attached  to  the  diameter  of  the 
barrel,  and  the  other  to  its  axis,  is  closely  coiled ;  but  as  the  ac- 
tion begins  on  the  smallest  part  of  the  fusee,  the  leverage  is 
short,  and  the  power  weak ;  but  as  the  fusee  turns,  and  the 
spring  uncoils,  the  leverage  increases  in  proportion  as  the  strength 
of  the  spring  becomes  weaker,  and  thus  the  two  forces  mutually 
equalize  each  other,  and  the  watch  runs  at  the  same  rate  until 
the  chain  which  connects  them  has  run  from  the  barrel  to  the 
fusee,  when  it  again  requires  winding,  and  the  same  process 
begins  again. 

350.  SYSTEM  OF  WHEELS. — As  the  wheel  and  axle  is  only  a 
modification  of  the  simple  lever,  so  a  system  of  wheels  acting  on 
each  other,  and  transmitting  the  power  to  the  resistance,  is  only 
another  form  of  the  compound  lever. 

351.  Such  a  combina- 
tion is  shown  in  Fig.  70. 
The    first    wheel,  A,  by 
means  of  the  teeth,  or  cogs, 
around  its  axle,  moves  the 
second  wheel,  B,  with  a 
force  equal  to  that  of  a 
lever,    the    long   arm    of 
which    extends    from  the 
center  to  the  circumference 
of  the  wheel,  where  the 
power  P  is  suspended,  and 
the  short    arm  from  the 
same  center  to  the  ends  of 
the  cogs.     The  dotted  line 
C,  passing  through  the  cen- 
ter of  the  wheel  A,  shows  the  position  of  the  lever,  as  the  wheel 
now  stands.     The  center  on  which  the  wheel  and  axle  turns,  is 
the  fulcrum  of  this  lever.     As  the  wheel  turns,  the  short  arm 

349.  ITow  does  the  fusee  equali/.e  this  ferce  1  Explain  how  the  forces  of  the  spring 
and  fusee  mutually  equalize  each  other.  350.  On  what  principle  does  a  system  of 
wheels  act,  as  represented  in  Fig.  701  351.  Explain  Fig.  70,  and  show  how  the 
power  P  is  transferred  by  the  action  of  levers? 


System  of  Wheels. 


86  WHEEL    AND    AXLE. 

of  this  lever  will  act  upon  the  long  arm  of  the  next  lever  by 
means  of  the  teeth  on  the  circumference  of  the  wheel  B,  and  this 
again  through  the  teeth  on  the  axle  of  B,  will  transmit  its  force 
to  the  circumference  of  the  wheel  D,  and  so  by  the  short  arm 
of  the  third  lever  to  the  weight  W.  As  the  power  or  small 
weight  falls,  therefore,  the  resistance  W,  is  raised,  with  the  mul- 
tiplied force  of  three  levers  acting  on  each  other. 

352.  In  respect  to  the  force  to  be  gained  by  such  a  machine, 
suppose  the  number  of  teeth  on  the  axle  of  the  wheel  A  to  be 
six  times  less  than  the  number  of  those  on  the  circumference  of 
the  wheel  B,  then  B  would  only  turn  round  once,  while  A  turns 
six  times.     And,  in  like  manner,  if  the  number  of  teeth  on  the 
circumference  of  D,  be  six  times  greater  than  those  on  the  axle 
of  B,  then  D  would  turn  once,  while  B  is  turned  six  times.     Thus 
six  revolutions  of  A  would  make  B  revolve  once,  and  six  revolu- 
tions of  B  would  make  D  revolve  once.     Therefore,  A  makes 
thirty-six  revolutions  while  D  makes  only  one. 

353.  The  diameter  of  the  wheel  A,  being  three  times  the 
diameter  of  the  axle  of  the  wheel  D,  and  its  velocity  of  motion 
being  36  to  1,  3  times  36  will  give  the  weight  which  a  power 
of  1  pound  at  P  would  raise  at  W.     Thus  36  X  3  =  108.     One 
pound  at  P  would  therefore  balance  108  pounds  at  W. 

354.  No   MACHINE   CREATES  FORCE. — If  the   student  has 
attended  closely  to  what  has  been  said  on  mechanics,  he  will 
now  be  prepared  to  understand,  that  no  machine,  however 
simple  or  complex,  can  create  the  least  degree  of  force.     It  is 
true,  that  one  man  with  a  machine  may  apply  a  force  which  a 
hundred  could  not  exert  with  their  hands,  but  then  it  would 
take  him  a  hundred  times  as  long. 

355.  Suppose  there  are  20  blocks  of  stone  to  be  moved  a 
hundred  feet ;  perhaps  twenty  men,  by  taking  each  a  block, 
would  move  them  all  in  a  minute.     One  man,  with  a  capstan, 
we  will  suppose,  may  move  them  all  at  once,  but  this  man,  with 
his  lever,  would  have  to  make  one  revolution  for  every  foot  he 
drew  the  whole  load  toward  him,  and  therefore  to  make  one 
hundred  revolutions  to  perform  the  whole  work.     It  will  also 
take  him  twenty  times  as  long  to  do  it,  as  it  took  the  twenty 
men.     His  task,  indeed,  would  be  more  than  twenty  times 
harder  than  that  performed  by  the  twenty  men,  for,  in  addition 
to  moving  the  stone,  he  would  have  the  friction  of  the  machinery 

353.  What  weight  will  one  pound  at  P  balance  at  W?  354.  Ts  there  any  actual 
power  framed  by  the  use  of  machinery?  355.  Suppose  twenty  men  to  move  twenty 
Btonestoa  certain  distance  with  their  hands,  and  one  man  moves  them  back  to  the 
same  place  with  a  capstan,  which  performs  the  most  actual  labor  ?  Why  ? 


WHEEL    AND    AXLE.  87 

to  .overcome,  which  commonly  amounts  to  nearly  one  third  of 
the  force  employed. 

356.  Hence  there  would  be  an  actual  loss  of  power  by  the 
use  of  the  capstan,  though  it  might  be  a  convenience  for  the 
one  man  to  do  his  work  by  its  means,  rather  than  to  call  in 
nineteen  of  his  neighbors  to  assist  him. 

357.  Any  power  by  which  a  machine  is  moved,  must  be 
equal  to  the  resistance  to  be  overcome,  and,  in  all  cases  where 
the  power  descends,  there  will  be  a  proportion  between  the 
velocity  with  which  it  moves  downward,  and  the  velocity  with 
which  the  weight  moves  upward. 

358.  There  will  be  no  difference  in  this  respect,  whether  the 
machine  be  simple  or  compound,  for  if  its  force  be  increased  by 
increasing  the  number  of  levers,  or  wheels,  the  velocity  of  the 
moving  power  must  also  be  increased,  as  that  of  the  resistance 
is  diminished. 

359.  There  being,  then,  always  a    proportion  between  the 
velocity  with  which  the  moving  force  descends,  and  that  with 
which  the  weight  ascends,  whatever  this  proportion  may  be,  it 
is  necessary  that  the  power  should  have  to  the  resistance  the 
same  ratio  that  the  velocity  of  the  resistance  has  to  the  velocity 
of  the  power.     In  other  words,  "  The  power  multiplied  by  the 
space  through  which  it  moves,  in  a  vertical  direction,  must  be 
equal  to  the  weight  multiplied  by  the  space  through  which  it 
moves  in  a  vertical  direction" 

This  law  is  known  under  FIG.  71. 

the  name  of  "the  law  of 
virtual  velocities,"  and  is  con- 
sidered the  golden  rule  of 
mechanics. 

360.  This  principle  has  al- 
ready been  explained,  while         / 
treating  of  the  lever,  (312  ;) 

but  that  the  student  should 
want  nothing  to  assist  him  in 
clearly  comprehending  so  im- 
portant a  law,  we  will  again 

illustrate   it    in   a   different  Weight  and  Space. 

manner. 

356.  Why,  then,  is  machinery  a  convenience?  357  In  the  use  of  the  lever,  what 
proportion  is  there  between  the  force  of  the  short  arm,  and  the  velocity  of  the  long 
arm  ?  Is  it  said,  tnat  the  velocity  of  the  power  downward,  must  be  in  proportion  to 
that  of  the  weurht  upward  ?  35S  Doe*  it  make  any  difference,  in  this  respect, 
whether  the  machine  be  simple  or  compound  1  359.  What  is  the  golden  rule  of  me 
chanics  ?  Explain  Fig.  71,  and  show  how  the  rule  is  illustrated  by  it. 


88  PULLEY. 

Suppose  the  lever,  Fig.  71,  to  be  thirty  inches  long  from  the 
fulcrum  to  the  point  where  the  power,  P,  is  suspended,  and 
that  the  weight.  W,  is  two  inches  from  the  fulcrum.  If  the 
power  be  1  pound,  the  weight  must  be  15  pounds,  to  produce 
equilibrium,  and  the  power,  P,  must  fall  thirty  inches  to  raise 
the  weight,  W,  two  inches.  Therefore,  the  power  being  1 
pound,  and  the  space  30  inches,  30  x  1  =30.  The  weight  being 
15  pounds,  and  the  space  2  inches,.  15  X  2  =30. 

Thus,  the  power  multiplied  by  the  space  through  which  it 
falls,  and  the  weight  multiplied  by  the  space  through  which 
it  rises,  are  equal. 

361.  However  complex  the  machine  may  be,  by  which  the 
force  of  a  descending  power  is  transmitted  to  the  weight  to  be 
raised,  the  same  rule  will  apply  as  it  does  to  the  action  of  the 
simple  lever. 

THE    PULLEY. 

362.  A  pulley  consists  of  a  wheel  which  is  grooved  on  the 
edge,  and  which  is  made  to  turn  on  its  axis,  by  a  cord  passing 
over  it. 

363.  Simple  Pulley. — Fig.  72,repre- 
sents  a  simple  pulley,  with  a  single  fixed 
wheel.     In  other  forms  of  the  machine, 
the  wheel  moves  up  and  down  with  the 
weight. 

364.  The  pulley  is  arranged  among 
the  simple  mechanical  powers ;  but  when 
several  are  connected,  the  machine  is 
called  a  system  of  pulleys,  or  a  com- 
pound pulley. 

365.  One  of  the  most  obvious  ad  van-  simple  Pulley 
tages,  of  the  pulley  is,  its  enabling  men 

to  exert  their  own  power  in  places  where  they  can  not  go  them- 
selves. Thus,  by  means  of  a  rope  and  wheel,  a  man  can  stand 
on  the  deck  of  a  ship,  and  hoist  a  weight  to  the  topmast. 

366.  By  means  of  two  fixed  pulleys,  a  weight  may  be  raised 
upward,  while  the  power  moves  in  a  horizontal  direction.     The 
weight  will  also  rise  vertically  through  the  same  space  that  the 
rope  is  drawn  horizontally. 

361.  What  is  said  of  the  application  of  this  rule  to  complex  machines'?  362.  What 
is  a  pulley  1  363.  What  is  a  simple  pulley  1  364.  What  is  a  system  of  pulleys,  or  a 
compound  pulley  1  365.  What  is  the  most  obvious  advantage  of  the  pulley?  366 
How  must  two  fixed  pulleys  be  placed  to  raise  a  weight  vertically  as  far  as  the 
power  goes  horizontally  ? 


PULLEY. 


89 


Fig.  73,  represents  two  fixed 
pulleys,  as  they  are  arranged  for 
such  a  purpose.  In  the  erection 
of  a  lofty  edifice,  suppose  the  up- 
per pulley  to  be  suspended  to 
some  part  of  the  building  ;  then 
a  horse  pulling  at  the  rope,  A, 
would  raise  the  weight,  W,  ver- 
tically, as  far  as  he  went  hori- 
zontally. 

367*  In  the  use  of  the  wheel 
of  the  pulley,  there  is  no  mechan- 
ical advantage,  except  that  which 
arises  from  removing  the  friction, 


FIG.  73. 


Simple  PuOey. 


FIG 


and  diminishing  the  imperfect  flexibility  of  the  rope. 

In  the  mechanical  effects  of  this  machine,  the  result  would 
be  the  same  did  it  slide  on  a  smooth  surface  with  the  same 
ease  that  its  motion  makes  the  wheel  revolve. 

368.  The  action  of  the  pulley  is  on  a  dif- 
ferent principle  from  that  of  the  wheel  and 
axle.     A  system  of  wheels,  as  already  ex- 
plained, acts  on  the  same  principle  as  the 
compound  lever.     But  the  mechanical  effica- 
cy of  a  system  of  pulleys  is  derived  entirely 
from  the  division  of  the  weight  among  the 
strings  employed  in  suspending  it    - 

369.  In  the  use  of  the  single  fixed  pul- 
ley,   there   can   be    no    mechanical    advan- 
tage, since  the  weight  rises  as  fast  as  the' 
power  descends.     This  is  obvious  by  Fig. 
74,  where  it  is  also  apparent  that  the  power 
and  weight  must  be  equal,  to  balance  each 
other,  as  already  shown. 

In  the  single  movable  pulley,  Fig.  74,  the 
same  rope  passes  from  the  fixed  point,  A,  to 
the  power,  P.  It  is  evident  here,  that  the  weight  is  supported 
equally  by  the  two  parts  of  the  string  between  which  it  hangs. 
Therefore,  if  we  call  the  weight,  W,  ten  pounds,  five  pounds 
will  be  supported  by  one  string,  and  five  by  the  other.  The 
power,  then,  will  support  twice  its  own  weight ;  so  that  a  per- 

367.  What  is  the  advantage  of  the  wheel  of  the  pulley  ?  368.  How  does  the  action 
of  the  pulley  differ  from  that  of.the  wheel  and  axle"?  369  Is  there  any  mechanical 
advantage  in  the  fixed  pulley  1  What  weight  at  P,  Fig.  74,  will  balance  ten  pounds 
at  W? 


Movable  Pulley. 


PULLET. 


son  pulling  with  a  force  of  five  pounds  at  P,  will  raise  ten 
pounds  at  W.  The  mechanical  force,  therefore,  in  respect  to  the 
power,  is  as  two  to  one. 

In  this  example,  it  is  supposed  there  are  only  two  ropes,  each 
of  which  bears  an  equal  part  of  the  weight. 


FIG.  75. 


FIG.  76. 


Compound  Pulley. 


System  of  Putteys. 


370.  Compound  Pulley. — If  the  number  of  ropes  be  in- 
creased, the  weight  may  be  increased  with  the  same  power ;  or 
the  power  may  be  diminished  in  proportion-  as  the  number  of 
ropes  is  increased.  In  Fig.  75,  the  number  of  ropes  sustaining 
the  weight  is  four,  and  therefore,  the  weight  may  be  four  times 
as  great  as  the  power.  This  principle  must  be  evident,  since  it 
is  plain  that  each  rope  sustains  an  equal  part  of  the  weight. 
The  weight  may,  therefore,  be  considered  as  divided  into  four 
parts,  and  each  part  sustained  by  one  rope. 


370.  Suppose  the  number  of  ropes  be  increased,  and  the  weight  increased,  must  the 
power  be  increased  also  ?  Suppose  the  weight,  Fig.  75,  to  be  thirty-two  pounds, 
what  will  each  rope  bear  7 


PULLEY.  91 

371.  In  Fig.  76,  there  is  a  system  of  pulleys  represented,  in 
which  the  weight  is  sixteen  times  the  power. 

The  tension  of  the  rope,  D  E,  is  evidently  equal  to  the  power, 
P,  because  it  sustains  it.  D,  being  a  movable  pulley,  must  sus- 
tain a  weight  equal  to  twice  the  power ;  but  the  weight  which 
it  sustains,  is  the  tension  of  the  second  rope,  D  C.  Hence,  the 
tension  of  the  second  rope  is  twice  that  of  the  first ;  and,  in 
like  manner,  the  tension  of  the  third  rope  is  twice  that  of  the 
second,  and  so  on,  the  weight  being  equal  to  twice  the  tension 
of  the  last  rope. 

372.  Suppose  the  weight,  W,  to  be  sixteen  pounds ;  then  the 
two  ropes,  8  and   8,  would  sustain  8  pounds  each,  this  being 
the  whole  weight  divided  equally  between  them.     The  next 
two  ropes,  4  and  4,  would  evidently  sustain  but  half  this  whole 
weight,  because  the  other  half  is  already  sustained  by  a  rope 
fixed  at  its  upper  end.     The  next  two  ropes  sustain  but  half  of 
4,  for  the  same  reason ;  and  the  next  pair,  1  and  1,  for  the 
same  reason,  will  sustain  only  half  of  2.     Lastly,  the  power,  P, 
will  balance  two  pounds,  because  it  sustains  but  half  this  weight, 
the  other  half  being  sustained  by  the  same  rope,  fixed  at  its 
upper  end. 

It  is  evident  that,  in  this  system,  each  rope  and  pulley  which 
is  added  will  double  the  effect  of  the  whole.  Thus,  by  adding 
another  rope  and  pulley  beyond  8,  the  weight,  W,  might  be  32 
pounds,  instead  of  16,  and  still  be  balanced  by  the  same  power. 

373.  In  our  calculations  of  the  effects  of  pulleys,  we  have 
allowed  nothing  for  the  weight  of  the  puHeys  themselves,  or  for 
the  friction  of  the  ropes.     In  practice,  however,  it  will  be  found 
that  nearly  one-third  must  be  allowed  for  friction,  and  that  the 
power,  therefore,  to  actually  raise  the  weight  must  be  about 
one- third  greater  than  has  been  allowed. 

374.  The  pulley,  like  other  machines,  obeys  the  laws  of 
virtual  velocities,  already  applied  to  the  lever  and  wheel.     Thus, 
"  in  a  system  of  pulleys,  the  ascent  of  the  weight,  or  resistance, 
is  as  muck  less  than  the  descent  of  the  power  as  the  weight  is 
greater  than  the  poioer"     If,  as  in  the  last  example,  the  weight 
is  1 6  pounds,  and  the  power  1  pound,  the  weight  will  rise  only 
1  foot,  while  the  power  descends  16  feet. 


371.  Explain  Fi?.  76,  and  show  what  part  of  the  weight  each  rope  pustains.  and 
why  one  pound  at  P,  will  balance  sixteen  pounds  at  W.  372.  Explain  the  reason 
why  each  additional  rope  and  pulley  will  double  the  effect  of  the  whole,  or  why  its 
weight  may  be  double  that  of  all  the  others  with  the  same  power.  373.  In  compound 
machines,  how  much  of  the  power  must  be  allowed  for  the  friction  1  374.  What 
general  law  applies  to  the  pulley  7 


92 


PULLED. 


375.  In  the  single  fixed  pulley,  the  weight  and  power  are 
equal,  and,  consequently,  the  weight  rises  as  fast  as  the  power 
descends. 

With  such  a  pulley,  a  man  may  raise  himself  up  to  the  mast- 
head by  his  own  weight.  Suppose  a  rope  is  thrown  over  a 
pulley,  and  a  man  ties  one  end  of  it  round  his  body,  and  takes 
the  other  end  in  his  ha'nds  ;  he  may  raise  himself  up,  because, 
by  pulling  with  his  hands,  he  has  the  power  of  throwing  more 
of  his  weight  on  that  side  than  on  the  other,  and  when  he  does 
this,  his  body  will  rise.  Thus,  although  the  power  and  the 
weight  are  the  same  individual,  still  the  man  can  change  his 
center  of  gravity  so  as  to  make  the  power  greater  than  the 
weight,  or  the  weight  greater  than  the  power,  and  thus  can 
elevate  one  half  of  his  weight  in  succession. 


WHITE  8   PULLEY. 

376.  In  all  the  pulleys  we  have  described, 
there  is  a  great  detect,  in  consequence  of  the 
different  velocities  at  which  the  several  wheels 
turn,  and  the  consequent  friction  to  which 
some  of  them  are  subjected. 

377.  It  has  been  an  object  among  mechan- 
ical philosophers,  to  remedy  this  defect  by 
inventing  a  system  of  pulleys,  the  wheels  of 
which  should  all  revolve  on  their  axles  in  the 
same  time,  each  making  the  same  number  of 
revolutions,     notwithstanding    the    different 
lengths  of  rope  passing  over  them,  and  thus 
avoid  a  defect  common  to  those  in  use. 

378.  This  object  seems  to  have  been  fully 
attained  by  Mr.  James  White,  whose  inven- 
tion is  represented  by  Fig.  77,  and  which  will 
be  understood  by  the  following  description. 
In  order  that  the  successive  wheels  should  re- 
volve in  the  same  time,  and  their  circumfer- 
ences should  be  just  equal  to  the  length  of 
rope  passing  over  them,  Mr.   White  made 
them  all  of  different  diameters.     By  this  con- 
struction, although  the  length  of  rope  passing 
over  each  was  different,  yet  their  revolutions 
are  equal,  both  with  respect  to  time  and  num- 
ber. 

By  this   arrangement   all    the   friction   is 


FIG.  77. 


Whiles  Pulley 


INCLINED    PLANK. 


93 


Inclined  Plane. 


avoided,  except  that  of  a  pivot  at  each  end,  and  the  lateral  fric- 
tion of  a  single  wheel.  A  single  rope  sustains  the  whole,  and  as 
in  other  systems,  the  weight  is  as  many  times  the  power  as  there 
are  ropes  sustaining  the  lower  block.  This  is  considered  the 
most  perfect  system  of  pulleys  yet  inrented. 

THE    INCLINED    PLANE. 

379.  This  power,  the  most  simple  of  all  machines,  consists  of 
a  hard,  smooth  plane,  inclined  to  the  horizon  in  various  degrees. 

It  is  the  fourth  me- 
chanical power,  and  is  FIG-  78- 
represented  by  Fig.  78, 
where  from  A  to  B  is 
the  inclined  plane  ;  the 
line  from  D  to  A,  is  its 
height,  and  that  from 
B  to  D,  its  base. 

A  board  with  one  end 

on  the  ground,  and  the  other  resting  on  a  block,  becomes  an 
inclined  plane. 

380.  This  machine  being  both  useful  and  easily  constructed, 
is  in  very  general  use,  especially  where  heavy  bodies  are  to  be 
raised  only  to  a  small  height.     Thus  a  man,  by  means  of  an 
inclined  plane,  which  he  can  readily  construct  with  a  board,  or 
couple  of  bars,  can  raise  a  load  into  his  wagon,  which  ten  men 
could  not  lift  with  their  hands. 

381.  The  power  required  to  force  a  given  weight  up  an  in- 
clined plane,  is  in  proportion  to  its  height,  and  the  length  of  its 
base,  or,  in  other  words,  tJte  force  must  be  in  proportion  to  the 
rapidity  of  its  inclination. 

382.  The  power,  P,  nc-  79- 
Fig.  79,  pulling  a  weight 

up  the  inclined  plane, 
from  C  to  D,  only  raises 
it  in  an  oblique  direction 
from  E  to  D,  by  acting 
along  the  whole  length 

Of    the    plane.       If    the  Inc'ined  Plane. 


375.  How  may  a  man  raise  himself  up  by  means  of  a  rope  and  single  fixed  pulley  ? 
376.  What  is  a  great  defect  in  the  common  pulley?  377.  In  what  manner  is  it  said 
that  the  defect  with  respect  to  friction  might  be  remedied  ?  378.  Describe  White's 

£  alley,  and  show  how  the  defects  in  other  "pulleys  are  remedied  by  this.     379.  What 
an  inclined  plane?    330.  On  what  occasions  is  this  power  chiefly  used?    Suppose 
a  man  wants  to  put  a  barrel  ef  cider  into  his  wagon,  how  does  he  make  an  inclined 
plane  for  this  purpoee? 


94 


INCLINED    PLANE. 


Inclined  Plane. 


plane  be  twice  as  long  as  it  is  high,  that  is,  if  the  line  from  C 
to  D  be  double  the  length  of  that  from  E  to  D,  then  one  pound 
at  P  will  balance  two  pounds  any  where  between  D  and  C.  It 
is  evident,  by  a  glance  at  this  figure,  that  were  the  base  length- 
ened, the  height  from  E  to  D  being  the  same,  a  less  power 
at  P  would  balance  an  equal  weight  any  where  on  the  inclined 
plane ;  and  so,  on  the  contrary,  were  the  base  made  shorter, 
that  is,  the  plane  more  steep,  the  power  must  be  increased  in 
proportion. 

383.  Suppose  two  in-  FIG.  so. 
clined  planes,  Fig.   80, 

of  the  same  height,  with 

bases  of  different  lengths; 

then    the    weight    and 

power  will  be  to  each 

other  as  the  length  of 

the  planes.    If  the  length 

from  A  to  B  is  two  feet,  and  that  from  B  to  C  one  foot,  then 

two  pounds  at  D  will  balance  four  pounds  at  W,  and  so  in  this 

proportion,  whether  the  planes  be  longer  or  shorter. 

384.  The  same  principle,  with  respect  to  the  virtual  veloci- 
ties of  the  weight  and  power,  applies  to  the  inclined  plane,  in 
common  with  the  other  mechanical  powers. 

Suppose  the  inclin- 
ed plane,  Fig.  81,  to  FIG.  81. 
be  two  feet  from  A  to 
B,  and  one  foot  from 
C  to  B,  then,  as  we 
have  already  seen  by 
Fig.  79,  a  power  of 
one  pound  at  P,  would 
balance  a  weight  of 
two  pounds  at  W. 
Now,  in  the  fall  of  the 
power  to  draw  up  the 
weight,  it  is  obvious 
that  its  vertical  de- 
scent must  be  just  twice  the  vertical  ascent  of  the  weight ;  for 


Indined  Plane. 


381.  To  roll  a  given  weight  up  an  inclined  plane,  to  what  must  the  force  be  pro- 
portioned? 382.  Explain  Fig.  79.  383.  If  (he  length  of  the  long  plane,  Fig.  80,  be 
double  that  of  the  short  one,  what  must  be  the  proportion  between  the  power  and 
the  weight?  384.  What  is  said  of  the  application  of  the  law  of  virtual  velocities  to 
the  inclined  plane  ?  Explain  Fig.  81,  and  show  why  the  power  must  fall  twice  as  far 
as  the  weight  rises. 


WEDGE.  95 

the  power  must  fall  down  the  distance  from  A  to  B,  to  draw, 
the  weight  that  distance ;  but  the  vertical  height  to  which  the 
weight  W  is  raised,  is  only  from  C  to  B.  Thus  the  power,  be- 
ing two  pounds,  must  fall  two  feet,  to  raise  the  weight,  four 
pounds,  one  foot ;  and  thus  the  power  and  weight,  multiplied 
by  their  several  velocities,  are  equal. 

When  the  power  of  an  inclined  plane  is  considered  as  a  ma- 
chine, it  must  therefore  be  estimated  by  the  proportion  which 
the  length  bears  to  the  height ;  the  power  being  increased  in 
proportion  as  the  elevation  of  the  plane  is  diminished. 

385.  APPLICATION  TO  ROADS. — Hilly  roads  may  be  regarded 
as  inclined  planes,  and  kteds  drawn  upon  them  in  carriages, 
considered  in  reference  to  the  powers  which  draw  them,  are 
subject  to  all  the  conditions  which  we  have  stated,  with  respect 
to  inclined  planes. 

The  power  required  to  draw  a  load  up  a  hill,  is  in  proportion 
to  the  elevation  of  the  inclined  plane.  On  a  road  perfectly 
horizontal,  if  the  power  is  sufficient  to  overcome  the  friction, 
and  the  resistance  of  the  atmosphere,  the  carriage  will  move. 
But  if  the  road  rise  one  foot  in  lifteen,  besides  these  impedi- 
ments, the  moving  power  will  have  to  lift  one  fifteenth  part  of 
the  load. 

386.  Now,  where  is  there  a  section  of  country  in  which  the 
traveler  is  not  vexed  with  roads,  passing  straight  over    hills, 
when  precisely  the  same  distance  would  carry  him  around  them 
on  a  level  plane  ?     To  use  a  homely,  but  very  pertinent  illustra- 
tion, "  the  bale  of  a  pot  is  no  longer,  when  it  lies  down,  than 
when  it  stands  up."     Had  this  simple  fact  been  noticed,  and  its 
practical  bearing  carried  into  effect  by  road  makers,  many  a 
high  hill  would  have  been  shunned  for  a  circuit  around  its  base, 
and  many  a  poor  horse,  could  he  speak,  would  thank  the  wis- 
dom of  such  a  decision. 


THE    WEDGE. 


387.  The  next  simple  mechanical  power  is  the  wedge.  This 
instrument  may  be  considered  as  two  inclined  planes,  placed 
base  to  base. 

It  is  much  employed  for  the  purpose  of  splitting  or  dividing 
solid  bodies,  such  as  wood  and  stone. 


385.  How  do  the  principles  of  the  inclined  plane  apply  to  roads  1  386.  What  i& 
said  about  the  bale  of  a  pot,  as  applied  to  road  making?  387.  On  what  principle 
does  the  wedge  act  ?  In  what  case  is  this  power  useful  1 


96 


SCREW. 


.  388.  Fig.  82  represents  such  a  wedge  as  is  FIG-  82- 
usually  employed  in  cleaving  timber.  This  in- 
strument is  also  used  in  raising  ships,  and  pre- 
paring them  to  launch,  and  for  a  variety  of  other 
purposes.  Nails,  awls,  needles,  and  many  cut- 
ting instruments,  act,  more  or  less,  on  the  prin- 
ciple of  this  machine. 

389.  There  is  much  difficulty  in  estimating 
the  power  of  the  wedge,  since  this  depends  on 
the  force,  or  the  number  of  blows  given  it,  to- 
gether with  the  obliquity  of  its  sides.  A  wedge 
of  great  obliquity  would  require  hard  blows  to 
drive  it  forward,  for  the  same  reason  that  a 
plane,  much  inclined,  requires  much  force  to  roll 
a  heavy  body  up  it.  But  were  the  obliquity  of 
the  wedge,  and  the  force  of  each  blow  given,  still  it  would  be 
difficult  to  ascertain  the  exact  power  of  the  wedge  in  ordinary 
cases,  for,  in  the  splitting  of  timber  and  stone,  for  instance,  the 
divided  parts  act  as  levers,  and  thus  greatly  increase  the  power 
of  the  wedge.  Thus,  in  a  log  of  wood,  six  feet  long,  when  split 
one  half  of  its  length,  the  other  half  is  divided  with  ease,  be- 
cause the  two  parts  act  as  levers,  the  lengths  of  which  con- 
stantly increase,  as  the  cleft  extends  from  the  wedge. 


Wedge. 


THE    SCREW. 


FIG. 


390.  The  screw  is  the  sixth  and  last  simple  mechanical  power. 
It  may  be  considered  as  a  modification  of  the  inclined  plane,  or 
as  a  winding  wedge. 

391.  It  is  an  inclined  plane  run- 
ning spirally  round  a  spindle,  as  will 
be  seen  by  Fig.  83.     Suppose  a  to 
be  a  piece  of  paper,  cut  into  the  form 
of  an  inclined  plane  and  rolled  round 
the  piece  of  wood  d ;  its  edge  would 
form  the  spiral  line,  called  the  thread 
of  the  screw.     If  the  finger  be  placed 
between  the  two  threads  of  a  screw, 
and  the  screw  be  turned  round  once, 

the  finger  will  be  raised  upward  equal  to  the  distance  of  the  two 


Winding  Wedge. 


388.  What  common  instruments  act  on  the  principle  of  the  wedge?  389.  What 
difficulty  is  there  in  estimating  the  |  ower  of  the  wedge?  390.  On  what  principle 
does  the  screw  act  1  391.  How  is  it  shown  that  the  screw  is  a  modification  of  the  in- 
clined plane? 


SCREW.  97 

threads  apart.  In  this  manner,  the  finger  is  raised  up  the 
inclined  plane,  as  it  runs  round  the  cylinder. 

The  power  of  the  screw  is  transmitted  and  emp'oyed  by 
means  of  another  screw  called  the  nut,  through  which  t  passes. 
This  has  a  spiral  groove  running  through  it,  which  exactly  fits 
the  thread  of  the  screw. 

If  the  nut  is  fixed,  the  screw  itself,  on  turning  it  roun..1,  ad- 
vances forward  ;  but  if  the  screw  is  fixed,  the  nut,  when  tui'ied, 
advances  along  the  screw. 

392.  Fig.  84  represents  the  first  kind  of  screw,  being  such 
as  is  commonly  used  in  pressing  paper,  and  other  substances. 
The  nut,  N,  through  which  the  screw  passes,  answers  also  for 
one  of  the  beams  of  the  press.  If  the  screw  be  turned  to  u  e 
right,  it  will  advance  downward,  while  the  nut  stands  still. 


FIG.  84. 


Nut  Fixed. 


Screw  Fixed. 


393.  A  screw  of  the  second  kind  is  represented  by  Fig.  85. 
In  this,  the  screw  is  fixed,  while  the  nut,  N,  by  being  turned  by 
the  lever,  L,  from  right  to  left,  will  advance  down  the  screw. 

394.  In  practice,  the  screw  is  never  used  as  a  simple  me- 
chanical machine ;  the  power  being  always  applied  by  means 
of  a  lever,  passing  through  the  head  of  the  screw,  as  in  Fig. 
84,  or  into  the  nut,  as  in  Fig.  85. 

395.  POWER  OF  THE  SCREW. — The  screw  acts  with  the  corn- 
lined  power  of  the  inclined  plane  and  the  lever,  and  its  force  is 


392.  Explain  Fi£.  84.  Which  is  the  screw,  and  which  the  nut  ?  Which  way  must 
the  screw  be  turned  to  make  it  advance  through  the  nut.  1  393.  How  does  the  screw, 
Fig.  84,  differ  from  Fig.  85  ?  394.  Is  the  screw  ever  used  as  a  simple  machine  ]  By 
what  simple  power  is  it  moved  ?  395.  What  two  simple  mechanical  powers  ar« 
concerned  in  the  force  of  the  screw  ? 


08  SCREW. 

such  as  to  be  limited  only  by  the  strength  of  the  materials  of 
which  it  is  made. 

In  investigating  the  effects  of  this  machine,  we  must,  there- 
fore, take  into  account  both  these  simple  mechanical  powers,  so 
that  the  screw  now  becomes  really  a  compound  engine. 

396.  In  the  inclined  plane,  we  have  already  seen,  that  the 
less  it  is  inclined,  the  more  easy  is  the  ascent  up  it.     In  apply- 
ing the  same  principle  to  the  screw,  it  is  obvious,  that  the 
greater  the  distance  of  the  threads  from  each  other,  the  more 
rapid  the  inclination,  and  consequently,  the  greater  must  be 
the  power  to  turn  it,  under  a  given  weight.     On  the  contrary, 
if  t>  e  thread  inclines  but  slightly,  it  will  turn  with  less  power, 
fr    the  same  reason  that  a  man  can  roll  a  heavy  weight  up  a 
plane  but  little  inclined.     Therefore,  the  finer  the  screw,  or  the 
nearer  the  threads  to  each  other,  the  greater  will  be  the  pres- 
sure under  a  given  power. 

397.  Let  us- suppose  two  screws,  the  one  having  the  threads 
one  inch  apart,  and  the  other  half  an  inch  apart;  then  the 
force  which  the  first  screw  will  give  with  the  same  power  at  the 
lever,  will  be  only  half  that  given  by  the  second.     The  second 
screw  must  be  turne<i  twice  as  many  times  round  as  the  first,  to 
go  through  the  same  space ;  but  what  is  lost  in  velocity  is  gained 
in  power.     At  the  lever  of  the  first,  two  men  would  raise  a 
given  weight  to  a  given  height,  by  making  one  revolution ; 
while  at  the  lever  of  the  second,  one  man  would  raise  the  same 
weight  to  the  same  height,  by  making  two  revolutions. 

398.  It  is  apparent  that  the  length  of  the  inclined  plane,  up 
which  a  body  moves  in  one  revolution,  is  the  circumference  of 
the  screw,  and  its  height  the  interval   between  the  threads. 
The  proportion  of  its  power  would  therefore  be  "as  the  circum- 
ference of  the  screw,  to  the  distance  between  the  threads,  so  is 
the  weight  to  the  power.''' 

399.  By  this  rule  the  power  of  the  screw  alone  can  be  found  ; 
but  as  this  machine  is  moved  by  means  of  the  lever,  we  must 
estimate  its  force  by  the  combined  power  of  both.     In  this  case, 
the  circumference  described  by  the  end  of  the  lever  employed, 
is  taken,  instead  of  the  circumference  of  the  screw  itself.     The 
means  by  which  the  force  of  the  screw  may  be  found,  is  there- 
fore, by  multiplying  the  circumference  which  the  lever  describes 
by  the  power. 

396.  Why  does  the  nearness  of  the  threads  make  a  difference  in  the  force  of  the. 
screw?  307.  Suppose  one  screw,  with  its  threads  one  inch  apart,  and  another  half 
an  inch  apart,  what  will  be  their  difference  in  force?  398.  What  is  the  length  of  the 
inclined  plane,  up  whi*h  a  body  moves  by  one  revolution  of  the  screw  ? 


SCREW. 


400.  Thus,  "  the  power  multiplied  by  the  cirwnfe  rente  which  it 
describes,  is  equal  to  the  weight  or  resistance,^xuj$c$aj[  by  the 
distance    between    the    two    contiguous    threads^jjOiiite   the 
efficacy  of  the  screw  may  be  increased,  by  increasingrfte  length 
of  the'lever,  or  by  diminishing  the  distance  between  the  threads. 
If,  then,  we  know  the  length  of  the  lever,  the  distance  between 
the  threads,  and  the  weight  to  be  raised,  we  can  readily  calcu- 
late the  power ;  or,  the  power  being  given,  and  the  distance  of 
the  threads  and  the  length  of  the  lever  known,  we  can  estimate 
the  weight  the  screw  will  raise. 

401.  "Thus,  suppose  the  length  of  the  lever  to  be  forty  inches, 
the  distance  of  the  threads  one  inch,  and  the  weight  8000 
pounds ;  required,  the  power,  at  the  end  of  the  lever,  to  raise 
the  weight.  . 

The  lever  being  40  inches,  the  diameter  of  the  circle,  whicli 
the  end  describes,  is  80  inches.  The  circumference  is  a  little 
more  than  three  times  the  diameter,  but  we  will  call  it  just 
three  times.  Then,  80x3  =  240  inches,  the  circumference  of 
the  circle.  The  distance  of  the  threads  is  1  inch,  and  the  weight 
8000  pounds.  To  find  the  power,  multiply  the  weight  by  the 
distance  of  the  threads,  and  divide  by  the  circumference  of  the 
circle.  Thus, 

Circum.       In.          Weight.        Power. 

240    •    1   :  :  8000    :    33* 

The  power  at  the  end  of  the  lever  must  therefore  be  33* 
pounds.  In  practice,  this  power  would  require  to  be  increased 
about  one-third,  on  account  of  friction. 

402.  PERPETUAL  SCREW. —  The  force  of  the  screw  is  some- 
times employed  to  turn  a  wheel,  by  acting  on  its  teeth.     In  this 
case  it  is  called  the  perpetual  screw. 

403.  Fig.  86  represents  such  a  machine.     It  is  apparent, 
that  by  turning  the  crank  C,  the  wheel  will  revolve,  for  the 
thread  of  the  screw  passes  between  the  cogs  of  the  wheel.     By 
means  of  an  axle,  through  the  center  of  this  wheel,  like  the 
common  wheel  and  axle,  this  becomes  an  exceedingly  powerful 
machine,   but  like  all  other  contrivances  for  obtaining   great 
power,  its  effective  motion  is  exceedingly  slow.     It  has,  how- 
ever, some  disadvantages,  and  particularly  the  great  friction  be- 


400.  How  is  the  force  of  the  screw  estimated  ?  How  may  the  efficacy  of  the  screw 
be  increased  ?  401.  The  length  of  the  lever,  the  distance  between  the  threads,  and 
the  wei«ht  being  known,  how  can  the  power  be  found?  Give  an  example.  402. 
What  is  the  screw  called  when  it  is  employed  to  turn  a  wheel?  403.  Explain  Fig 
86.  What  is  the  objection  to  this  machine  for  raising  weights? 


100 


SCREW. 


tween  the  thread  of  the  screw  and  FIG.  86. 

the  teeth  of  the  wheel,  which  pre- 
vents it  from  being  generally  em- 
ployed to  raise  weights. 

404.  ALL  THESE  MECHANICAL 

POWERS  RESOLVED  INTO  THREE. 

We  have  now  enumerated  and  de- 
scribed all  the  mechanical  powers 
usually  denominated  simple.  They 
are  six  in  number,  namely,  the 
Lever,  Wheel  and  Axle,  Pulley, 
Wedge,  Inclined  Plane,  and  Screw.  screw  and  Wheel. 

405.  In  respect  to  the  principles 

o-n  which  they  act,  they  may  be  resolved  into  three  simple 
powers,  namely,  the  lever,  the  inclined  plane,  and  the  pulley ; 
for  it  has  been  shown  that  the  wheel  and  axle  is  only  another 
form  of  the  lever,  and  that  the  screw  is  but  a  modification  of 
the  inclined  plane. 

It  is  surprising,  indeed,  that  these  simple  powers  can  be  so 
arranged  and  modified,  as  to  produce  the  different  actions  in  all 
that  vast  variety  of  intricate  machinery  which  men  have  in- 
vented and  constructed. 

406.  CARD  MACHINE. — The  variety  of  motions  we  witness  in 
the  little  engine  which  makes  cards,  by  being  supplied  with 
wire  for  the  teeth,  and  strips  of  leather  to  stick  them  through, 
would  itself  seem  to  involve  more  mechanical  powers  than  those 
enumerated.     This  engine  takes  the  wire  from  a  reel ;  bends  it 
into  the  form  of  teeth ;  cuts  it  off;  makes  two  holes  in  the 
leather  for  the  tooth  to  pass  through ;  sticks  it  through ;  then  gives 
it  another  bend  on  the  opposite  side  of  the  leather ;  graduates 
the  spaces  between  the  rows  of  teeth,  and  between  one  tooth 
and  another ;  and,  at  the  same  time,  carries  the  leather  back- 
ward and  forward,  before  the  point  where  the  teeth  are  intro- 
duced, with  a  motion  so  exactly  corresponding  with  the  motions 
of  the  parts  which  make  and  stick  the  teeth,  as  not  to  produce 
the  difference  of  a  hair's  breadth  in  the  distance  between  them. 

All  this  is  done  without  the  aid  of  human  hands,  any  further 
than  to  put  the  leather  in  its  place,  and  turn  a  crank ;  or,  in 
some  instances,  many  of  these  machines  are  turned  at  once,  by 
means  of  three  or  four  dogs,  walking  on  an  inclined  plane  which 
revolves. 


404.  How  many  simple  mechanical  powers  are  there,  and  what  are  they  called  7 
405.  How  can  they  be  resolved  into  three  simple  powers  1  406.  What  is  said  of  the 
card-making  machine  ? 


SCREW.  101 

407.  Such  a  machine  displays  the  wonderful  ingenuity  and 
perseverance  of  man,  and  at  first  sight  would  seem  to  set  at 
naught  the  idea  that  the  lever  and  wheel  are  the  chief  simple 
powers  concerned  in  its  motions.     But  when  these  motions  are 
examined  singly  and  deliberately,  we  are  soon  convinced  that 
the  wheel  variously  modified,  is  the  principal  mechanical  power 
in  the  whole  engine." 

408.  USE  OF  MACHINERY. — It  has  already  been  stated,  (354,) 
that  notwithstanding  the  vast  deal  of  time  and  ingenuity  which 
men  have  spent  on  the  construction   of  machinery,  and   in 
attempting  to  multiply  their  powers,  there  has,  as  yet,  been 
none  produced,  in  which  the  power  was  not  obtained  at  the 
expense  of  velocity,  or  velocity  at  the  expense  of  power ;  and, 
therefore,  no  actual  force  is  ever  generated  by  machinery. 

When  men  employ  the  natural  elements  as  a  power  to  over- 
come resistance  by  means  of  machinery,  there  is  a  vast  saving 
of  animal  labor.  Thus  mills,  and  all  kinds  of  engines,  which 
are  kept  in  motion  by  the  power  of  water,  or  wind,  or  steam, 
save  animal  .labor  equal  to  the  power  it  takes  to  keep  them  in 
motion. 

409.  Fine  Mechanical  Powers  in  one  Machine. — An  engineer, 
it  is  said,  for  the  purpose  of  drawing  a  ship  out  of  the  water  to 
be  repaired,  combined  the  mechanical  powers  represented  by 
Fig.  87,  and  perhaps  no  machine  ever  constructed  gives  greater 
force  with  so  small  a  power. 


FIG.  87. 


The  Fire  Mechanical  Powers. 


It  involves  the  lever  A,  wheel  and  axle  B,  the  pulley  C,  the 
inclined  plane  D,  and  the  screw  E. 

407.  What  are  the  chief  mechanical  powers  concerned  in  its  motions  1  408.  Is 
there  any  actual  force  generated  by  machinery  I  What  is  said  of  employing  the 
natural  elements  as  a  power  1  409.  What  are  the  five  mechauieal  powers  employed 
hi  Fig.  87 1  Point  out  on  the  cut  the  place  of  each  nower. 


102  HYDROSTATICS. 

To  estimate  the  force  of  this  engine,  it  is  necessary  to  know 
the  length  of  the  lever,  diameter  of  the  wheel,  &c. 

Suppose  then,  the  sizes  of  the  different  powers  are  as  fol- 
lows, viz. : — 

Length  of  the  lever  A, 18  inches. 

Distance  of  the  thread  E, 1  inch. 

Diameter  of  the  wheel  B, 4  feet. 

Diameter  of  the  axle, 1  foot. 

Pulleys  C  and  D,  D  fixed, 4  strings. 

Height  of  the  plane  D,  one-half  its  length, .     .  2  feet. 

Suppose  the  man  turns  the  lever  A,  with  the  power  equal  to 
100  pounds,  the  force  on  the  ship  would  thus  be  found,  for  the 
different  laws  and  rules  referring  to  each  mechanical  power. 

1.  One  hundred  pounds  on  the  lever  A,  would  be- 
come a  force  by  means  of  the  screw  on  the  wheel      Pounds. 

B  of 11,309.76 

2.  Diameter  of  wheel  four  times  that  of  the  axle,  .  4 

45,239.04 

3.  The  number  of  pulley  strings, 4 

180,956.16 

4.  Height  of  the  inclined  plane  half  its  lenth,  .     .  2 

361,912.32 

The  force  on  the  ship  therefore  would  be  equal  to  361,912 
pounds,  or  about  161  tons. 


CHAPTER  V. 

HYDROSTATICS. 

410.  Hydrostatics  is  the  science  which  treats  of  the  weight, 
pressure,  and  equilibrium  of  water,  or  other  fluids,  when  in  a 
state  of  rest. 

411.  Hydraulics  is  that  part  of  the  science  of  fluids  which 
treats  of  water  in  motion,  and  the  means  of  raising  and  con- 
ducting it  in  pipes,  or  otherwise,  for  all  sorts  of  purposes. 

409.  What  must  be  known  to  estimate  the  power  of  this  machine?  What  is  the 
amount  of  force  on  the  ship?  410.  What  is  hydrostatics  1  411.  How  does  hydraulics 
differ  from  hydrostatics  1 


HYDROSTATICS.  103 

The  subject  of  water  at  rest,  will  first  claim  investigation, 
since  the  laws  which  regulate  its  motion  will  be  best  understood 
by  first  comprehending  those  which  regulate  its  pressure. 

412.  A  fluid  is  a  substance  whose  particles  are  easily  moved 
among  each  other,  as  air  and  water. 

413.  The  air  is  called  an  elastic  fluid,  because  it  is  easily 
compressed  into  a  smaller  bulk,  and  returns  again  to  its  original 
state  when  the  pressure  is  removed.     Water  is  called  a  non- 
elastic  fluid,  because  it  admits  of  little  diminution  of  bulk  under 
pressure. 

414.  The  non-elastic  fluids  are  perhaps  more  properly  called 
liquids,  but  both  terms  are  employed  to  signify  water  and  othei 
bodies  possessing  its  mechanical  properties.     The  term  fluid, 
when  applied  to  the  air,  has  the  word  elastic  before  it. 

415.  One  of  the  most  obvious  properties  of  fluids,  is  the 
facility  with  which  they  yield  to  the  impressions  of  other  bodies, 
and  the  rapidity  with  which  they  recover  their  former  state, 
when  the  pressure  is  removed.     The  cause  of  this,  is  the  free- 
dom with  which  their  particles  slide  over, or  among  each  other; 
their  cohesive  attraction  being  so  slight  as  to  be  overcome  by 
the  least  impression.     On  this  want  of  cohesion  among  their 
particles  seems  to  depend  the  peculiar  mechanical  properties  of 
these  bodies. 

416.  In  solids,  there  is  such  a  connection  between  the  parti- 
cles, that  if  one  part  moves,  the  other  part  must  move  also. 
But  in  fluids,  one  portion  of  the  mass  may  be  in  motion,  while 
the  other  is  at  rest.     In  solids,  the  pressure  is  always  downward, 
or  toward  the  center  of  the  earth's  gravity  ;  but  in  fluids,  the 
particles  seem  to  act  on  each  other  as  wedges,  and  hence,  when 
confined,  the  pressure  is  sideways,  and  even  upward,  as  well  as 
downward. 

417.  Elasticity  of  Water. — Water  has  commonly  been  called 
a  non-elastic  substance,  but  it  is  found  that  under  great  pressure 
its  volume  is  slightly  diminished,  and  hence  it  is  proved  to  be 
elastic.     The  most  decisive  experiments  on  this  subject  were 
made  many  years  ago  by  Mr.  Perkins. 

418.  These  experiments  were  made  by  means  of  a  hollow 
cylinder,  Fig.  88,  which  was  closed  at  the  bottom,  and  made 
water-tight  at  the  top,  by  a  cap,  screwed  on.     Through  this 

•  412.  What  is  a  fluid  1  413  What  is  an  elastic  fluid  ?  Why  is  air  called  an  elastic 
flu  (1  ?  414  What  substances  are  called  liquids?  415.  What  is  one  of  the  most  ob- 
vious properties  of  liquids?  416.  On  what  do  the  peculiar  mechanical  properties  of 
fluids  depend  ?  In  what  respect  does  the  pressure  of  a  fluid  diflVr  from  that  of  a 
solid  ?  417.  Is  water  an  elastic,  or  a  non-elastic  fluid  J  413.  Describe  Fig.  88,  and 
show  how  water  was  found  to  be  elastic. 


Water 


104  PRESSURE    OF    WATER. 

cap,  at  A,  passed  the  rod  B,  which  was  five-sixteenths       FIG.  s& 

of  an  inch  in  diameter.     The  rod  was  so  nicely  fitted 

to  the  cap,  as  also  to  be  water-tight.     Around  the 

rod  at  C,  there  was  placed  a  flexible  ring,  which  could 

be  easily  pushed  up  or  down,  but  fitted  so  closely  as 

to  remain  on  any  part  where  it  was  placed. 

A  cannon  of  sufficient  size  to  receive  this  cylinder, 
which  was  three  inches  in  diameter,  was  furnished 
with  a  strong  cap  and  forcing  pump,  and  set  verti- 
cally into  the  ground.  The  cannon  and  cylinder 
were  next  filled  with  water,  and  the  cylinder,  with 
its  rod  drawn  out,  and  the  ring  placed  down  to  the 
cap,  as  in  the  figure,  was  plunged  into  the  cannon. 
The  water  in  the  cannon  was  then  subjected  to  an 
immense  pressure  by  means  of  the  forcing  pump,  Elastic. 
after  which,  on  examination  of  the  apparatus,  it  was 
found  that  the  ring  C,  instead  of  being  where  it  was  placed,  w^i 
eight  inches  up  the  rod.  The  water  in  the  cylinder  being  com- 
pressed into  a  smaller  space,  by  the  pressure  of  that  in  the  can- 
non, the  rod  was  driven  in,  while  under  pressure,  but  was  forced 
out  again  by  the  expansion  of  the  water,  when  the  pressure 
was  removed.  Thus,  the  ring  on  the  rod  would  indicate  the 
distance  to  which  it  had  been  forced  in,  during  the  greatest 
pressure. 

419.  This  experiment  proved  that  water,  under  the  pressure 
of  one  thousand  atmospheres,  that  is,  the  weight  of  15,000 
pounds  to  the  square  inch,  was  reduced  in  bulk  about  one  part 
iir  24. 

So  slight  a  degree  of  elasticity  under  such  immense  pressure, 
is  not  appreciable  under  ordinary  circumstance,  and  therefore 
in  practice,  or  in  cpmrnon  experiments  on  this  fluid,  water  is 
considered  as  non-elastic. 

EQUAL   PRESSURE    OF   WATER. 

420.  The  particles  of  water,  and  other  fluids,  when  confined, 
press  on  the  vessel  which  confines  them,  in  all  directions,  both 
upward,  downward,  and  sideways. 

From  this  property  of  fluids,  together  with  their  weight,  very 
unexpected  and  surprising  effects  are  produced. 

The  effect  of  this  property,  which  we  shall  first  examine,  is, 


419.  In  what  proportion  does  the  bulk  of  water  diminish  under  a  pressure  of  15,000 
pounds  to  the  square  inch  1  In  common  experiments,  is  water  considered  elastic, 
or  non-elastic?  420.  When  water  is  confined,  in  what  direction  does  it  press  1 


PRESSURE    OF    WATER. 


105 


FIG.  89. 


that  a  quantity  of  water,  however  small,  will  balance  another 
quantity,  however  large.  Such  a  proposition  at  first  thought 
might  seem  very  improbable.  But  on  examination,  we  shall 
find  that  an  experiment  with  a  very  simple  apparatus  will  con- 
vince any  one  of  its  truth.  Indeed,  we  every  day  see  this  prin- 
ciple established  by  actual  experiment,  as  will  be  seen  directly. 

421.  Fig.   89,    represents  a    common 
coffee-pot,  supposed  to  be  filled  up  to  the 
dotted  line  A,  with  a  decoction  of  coffee, 
or  any  other  liquid.     The  coffee,  we  know, 
stands  exactly  at  the  same  height,  both  in 
the   body  of  the  pot,   and  in  its  spout. 
Therefore,  the  small  quantity  in  the  spout, 
balances  the  large  quantity  in  the  pot,  or 
presses  with  the  same  force  downward,  as 
that  in   the  body  of  the  pot  presses  up- 
ward.    This  is  obviously  true,  otherwise,   the  large  quantity 
would  sink  below  the  dotted  line,  while  that  in  the  spout  would 
rise  above  it,  and  run  over. 

422.  The  same  principle  is  more  strik- 
ingly illustrated  by  Fig.  90. 

Suppose  the  cistern  A  to  be  capable  of 
holding  one  hundred  gallons,  and  into  its 
bottom  there  be  fitted  the  tube  B,  bent, 
as  seen  in  the  figure,  and  capable  of  con- 
taining one  gallon.  The  top  of  the  cis- 
tern, and  that  of  the  tube,  being  open, 
pour  water  into  the  tube  at  C,  and  it  will 
rise  up  through  the  perpendicular  bend 
into  the  cistern,  and  if  the  process  be  con- 
tinued, the  cistern  will  be  filled  by  pour- 
ing water  into  the  tube.  Now  it  is  plain, 

that  the  gallon  of  water  in  the  tube  presses  against  the  hun- 
dred gallons  in  the  cistern,  with  a  force  equal  to  the  pressure  of 
the  hundred  gallons,  otherwise,  that  in  the  tube  would  be  forced 
upward  higher  than  that  in  the  cistern,  whereas,  we  find  that 
the  surfaces  of  both  stand  exactly  at  the  same  height. 

423.  From   these  experiments  we  learn,  "  that  the  pressure 
of  a  Jluid  is  not  in  proportion  to  its  quantity,  but  to  its  height, 


Coffee-Pot. 


FIG.  90. 


Pressure  of  Water. 


421.  now  does  the  experiment  with  the  coffee-pot  show  that  a  small  quantity  of 
liquid  will  balance  a  large  one!  422.  Explain  Fig.  90,  and  show  how  the  pressure 
in  the  tube  is  equal  to  the  pressure  in  the  cistern.  423.  What  conclusion,  or  gen- 
eral truth,  is  to  be  drawn  from  these  experiments  ? 

5* 


106 


PRESSURE    OF    WATER. 


and  that  a  large  quantity  of  water  in  an  open  vessel,  presses 
with  no  more  force  than  a  small  quantity  of  the  same  height?' 
424.  Pressure  equal  in  Vessels  of  all  Sizes  and  Shapes. — 
The  size  or  shape  of  a  vessel  is  of  no  consequence,  for  if  a  num- 
ber of  vessels,  differing  entirely  from  each  other  in  figure,  posi- 
tion, and  capacity,  have  a  communication  made  between  them, 
and  one  be  filled  with  water,  the  surface  of  the  fluid,  in  all,  will 
be  at  the  same  elevation.  If,  therefore,  the  water  stands  at  an 
e^ual  height  in  all,  the  pressure  in  one  must  be  just  equal  to 
that  in  another,  and  so  equal  to  that  in  all  the  others. 


Equal  Pressure  of  Water. 


425.  To  make  this  obvious,  suppose  a  number  of  vessels,  of 
different  shapes  and  sizes,  as  represented  by  Fig.  91,  to  have  a 
communication  between  them,  by  means  of  a  small  tube,  pass- 
ing from  the  one  to  the  other.     If,  now,  one  of  these  vessels  be 
filled  with  water,  or  if  water  be  poured  into  the  tube  A,  all  the 
other  vessels  will  be  filled  at  the  same  instant,  up  to  the  line 
B  C.     Therefore,  the  pressure  of  the  water  in  A,  balances  that 
in  1,  2,  3,  &c.,  while  the  pressure  in  each  of  these  vessels  is 
equal  to  that  in  the  other,  and  so  an  equilibrium  is  produced 
throughout  the  whole  series. 

426.  If  an  ounce  of  water  be  poured  into  the  tube  A,  it  will 
produce  a  pressure  on  the  contents  of  all  the  other  vessels,  equal 
to  the  pressure  of  all  the  others  on  the  tube :  for,  it  will  force 
the  water  in  all  the  other  vessels  to  rise  upward  to  an  equal 
height  to  that  in  the  tube  itself.     Hence,  we  must  conclude, 
that  the  pressure  in  each  vessel  is  not  only  equal  to  that  in  any 


424.  What  difference  does  the  shape  or  size  of  a  vessel  make  in  respect  to  the  pres- 
sure of  a  fluid  on  ite bottom?  425.  Explain  Fisr.  91,  and  show  how  the  equilibrium 
is  produced.  426.  Suppose  an  ounce  of  water  be  poured  into  the  tube  A,  what  will 
be  its  effect  on  th«  contents  of  the  other  vessels  1 


PRESSURE    OF    WATER. 


107 


FIG.  92. 


of  the  others,  but  also  th^t  the  pressure  in  any  one  is  equal  to 
that  in  all  the  others. 

427.  From  this,  we  learn  that  the  shape  or  size  of  a  vessel 
has  no  influence  on  the  pressure  of  its  liquid  contents,  but  that 
the  pressure  of  water  is  as  its  height,  whether  the  quantity  be 
great  or  small.     We  learn,  also,  that  in  no  case  will  the  weight 
of  a  quantity  of  liquid,  however  large,  force  another  quantity, 
however  small,  above  the  level  of  its  own  surface. 

428.  Now,  by  other  experiments,  it  is  ascertained,  that  the 
pressure  of  a  liquid  is  in  proportion  to  its  height,  and  the  area 
of  its  base. 

Suppose  a  vessel,  ten  feet  high,  and  two 
feet  in  diameter,  such  as  is  represented  at 
A,  Fig.  92,  to  be  filled  with  water ;  there 
would  be  a  certain  amount  of  pressure,  at 
C,  near  the  bottom.  Let  D  represent  an- 
other vessel,  of  the  same  diameter  at  the 
bottom,  but  only  a  foot  high,  and  closed 
at  the  top.  Now  if  a  small  tube,  the  fourth 
of  an  inch  in  diameter,  be  inserted  into  th,e 
cover  of  this  vessel,  and  the  tube  be  car- 
ried to  the  height  of  the  vessel  A,  and  then 
the  vessel  and  tube  be  filled  with  water, 
the  pressure  on  the  bottoms  and  sides  of 
both  vessels  at  the  same  height  will  be 
equal,  and  jets  of  water  starting  from  D 
and  C  will  have  exactly  the  same  force, 
and  spout  to  the  same  distance. 

This  might  at  first  seem  improbable,  but  to  convince  our- 
selves of  its  truth,  we  have  only  to  consider,  that  any  impres- 
sion made  on  one  portion  of  the  confined  fluid  in  the  vessel  D, 
is  instantly  communicated  to  the  whole  mass.  Therefore,  the 
water  in  the  tube  B,  presses  with  the  same  force  on  every  other 
portion  of  the  water  in  D,  as  it  does  on  that  small  portion  over 
which  it  stands. 

429.  Bursting  a   Cask. — This  principle  is  illustrated  in  a 
very  striking  manner,  by  the  experiment,  which  has  often  been 
made,  of  bursting  a  common  wine  cask  with  a  few  ounces  of 
water. 


427.  What  conclusion  is  to  be  drawn  from  pouring  the  ounce  of  water  into  the  tube 
A  1  What  is  the  reason  that  a  large  quantity  of  water  will  not  ibrce  a  small  quantity 
above  its  own  level  ?  Is  the  force  of  water  in  proportion  to  its  height,  or  its  quan- 
tity ?  428  How  is  a  small  quantity  of  water  shown  to  press  equal  to  a  large  quantity 
by  Fig.  92  7  429.  Explain  the  reason  why  the  pressure  is  as  great  at  D,  as  at  O. 


108 


PRESSURE    OF    WATER. 


FIG.  93. 


Suppose  A,  Fig.  93,  to  be  such  a  cask,  already 
filled  with  water,  and  suppose  the  tube  B,  fifty 
feet  high,  to  be  screwed,  water-tight,  into  'its 
head.  When  water  is  poured  into  the  tube, 
so  as  to  fill  it  gradually,  the  cask  will  show  in- 
creasing signs  of  pressure,  by  emitting  the 
water  through  the  pores  of  the  wood,  and  be- 
tween the  joints ;  and,  finally,  as  the  tube  is 
filled,  the  cask  will  burst  asunder. 

430.  The  same  apparatus  will  serve  to  illus- 
trate the  upward  pressure  of  water ;  for,  if  a 
small  stop-cock  be  fitted  to  the  upper  head,  on 
turning  this,  when  the  tube  is  filled,  a  jet  of 
water  will  spirt  up  with  a  force,  and  to  a  height, 
that  will  astonish  all  who  never  before  saw  such 
an  experiment. 

In  theory,  the  water  will  spout  to  the  same 
height  with  that  which  gives  the  pressure,  but,     Bursting  a  Cask 
in  practice,  it  is  found  to  fall  short  in  the  fol- 
lowing proportions : — 

431.  If  the  tube  be  twenty  feet  high,  and  the  orifice  for  the 
jet  half  an  inch  in  diameter,  the  water  will  spout  nearly  nine- 
teen feet.     If  the  tube  be  fifty  feet  high,  the  jet  will  rise  up- 
ward of  forty  feet,  and  if  a  hundred  feet,  it  will  rise  above  eighty 
feet.     It  is  understood,  in  every  case,  that  the  tubes  are  to  be 
kept  fall  of  water. 

The  height  of  these  jets  shows  the  astonishing  effects  that  a 
small  quantity  of  fluid  produces  when  pressing  from  a  perpen- 
dicular elevation. 

432.  HYDROSTATIC  PARADOX. — This  paradox,  illustrated  by 
Fig.  94,  consists  in  experimental  proof  of  the  principle  already 
insisted  on,  that  water  presses  according  to  its  height,  and  not 
to  its  quantity.     Fill  a  glass  jar  with  water,  and  balance  it  on 
the  scale-beam  F,  E,  with  small  weights.     Then  pour  out  the 
water,  leaving  only  an  inch  or  two  deep,  letting  the  balance 
weights   remain.     Replacing   the  jar,   which    will    now  stand 
higher  than  before,  owing  to  the  loss  of  water,  introduce  into 
it,  by  means  of  the  crane,  II,  a  piece  of  wood  a  few  lines  smaller 
in  all  directions  than  the  inside  of  the  jar.     The  wood  being 

How  is  the  same  principle  illustrated  by  Fig.  93  1    430.  How  may  Fijr.  93  be  made 
to  illustrate  the  upward  pressure  of  water!    431.  Under  the  pressure  of  a  column 
of  water  twenty  feet  high,  what  will  be  the  height  of  the  jet,  I    Under  a  pressure  of  a 
hundred  feet,  how  high  will  it  rise  ?    432.  What  does  the  hydrostatic  paradox  show 
Explain  by  the  figure  how  the  experiment  is  made. 


PRESSURE    OF    WATBB.  109 

FIG.  91 


Hydrostatic  Paradox. 

adjusted  by  means  of  the  thumb-screw,  so  that  the  water  is 
made  to  rise  around  it  exactly  to  the  brim,  or  as  high  as  it 
stood  before  any  was  poured  out,  (the  wood  not  touching  the 
glass,)  and  it  will  be  found  that  it  will  exactly  balance  the 
weights,  as  it  did  when  full  of  water,  though  it  now  contains 
only  a  tenth  as  much  as  before. 

The  result  will  be  the  same  if.  instead  of  the  wood,  the  same 
bulk  of  cork  or  lead  be  placed  in  the  jar,  the  only  point  being, 
that,  in  each  case,  the  water  should  rise  to  the  same  height. 

The  above  experiment  proves,  in  a  very  striking  manner,  that 
the  pressure  of  water  is  as  its  height ;  and  the  reason  why  it 
makes  no  difference  in  the  result  whether  the  body  placed  in 
the  jar  be  of  wood,  cork,  or  lead  is,  that  the  solid  merely  takes 
the  place  of  the  fluid,  displacing  its  own  bulk,  and  thus  the 
weight  remains  just  as  though  the  water  itself  had  remained  in 
the  jar.  Thus,  the  pressure  of  a  tenth  part  of  the  water,  of 
equal  height,  equals  the  whole. 

433.  Proof  by  Mercury. — In  addition  to  the  above  proofs, 
that  a  small,  will  balance  a  large  quantity  of  water,  we  add  the 
following,  perhaps  the  most  satisfactory  of  all. 

Let  A,  B,  C,  Fig.  95,  represent  a  glass  tube,  having  at  A,  a 
collar  cemented  to  the  glass,  into  which  vessels  of  different  ca- 
pacities and  shapes,  may  be  screwed.  The  tube  is  first  filled 
with  mercury  up  to  the  level  of  the  dotted  line  A  C,  and  the 
tube  G  jt?,  fitted  in  its  place.  The  vessel  D,  is  then  screwed 
into  A,  and  water  is  poured  in  as  far  as  A,  the  base  of  the  column 

433.  Explain  Fig.  95,  and  show  in  what  manner  different  quantities  of  water  will 
balance  the  same  weight  of  mercury. 


110 


PRESSURE    OF    WATER. 


Proof  by  Mercury. 


FIG.  96. 


of  water  being,  as  seen,  ''IQ- 

equal  to  that  of  the  mer- 
cury. The  fluid  metal 
will  rise,  by  the  pressure 
of  the  water  on  A,  up  to 
p  in  the  small  tube.  Then 
unscrew  D,  and  in  its 
place  fix  the  conical  ves- 
sel E,  and  pour  in  water 
as  before,  and  the  same 
result  will  follow,  and  so 
with  the  small  tube  F; 
in  each  case,  the  height 
of  the  water,  notwithstanding  the  difference  in  quantity,  will 
force  the  mercury  to  exactly  the  same  elevation. 

434.  HYDROSTATIC   BELLOWS. — An   instrument   called   the 
hydrostatic  bellows,  also  shows,  in  a  striking  manner,  the  great 
force  of  a  small  quantity  of  water,  pressing  in  a  perpendicular 
direction. 

This  instrument  consists  of  two 
boards,  connected  together  with  strong 
leather,  in  the  manner  of  the  common 
bellows.  It  is  then  furnished  with  a 
tube  A,  Fig.  96,  which  communicates 
between  the  two  boards.  A  person 
standing  on  the  upper  board  may  raise 
himself  up  by  pouring  water  into  the 
tube.  If  the  tube  holds  an  ounce  of 
water,  and  has  an  area  equal  to  a 
thousandth  part  of  the  area  of  the  top 
of  the  bellows,  one  ounce  of  water  in 
the  tube  will  balance  a  thousand  ounces 
placed  on  the  bellows. 

435.  HYDROSTATIC    PRESS.  —  This 
property  of  water  was  applied  by  Mr. 

Bramah,  to  the  construction  of  his  hydrostatic  .  press.  But 
instead  of  a  high  tube  of  water,  which  in  most  cases  could  not 
be  so  readily  obtained,  he  substituted  a  strong  forcing-pump, 
and  instead  of  the  leather  bellows,  a  metallic  pump,  barrel,  and 
piston. 

434.  What  is  the  hydrostatic  bellows!  What  property  of  water  is  this  instrument 
designed  to  show?  435.  Explain  Fig.  97.  Where  is  the  piston?  Which  is  the 
pump-barrel  in  which  it  wftrks  1 


Hydrostatic  Bellows. 


PRESSURE    OF    WATER. 


Ill 


Hydrostatic  Press. 


This    arrangement   will   be       .  FIG.  97. 

understood  by  Fig.  97,  where 
the  pump-barrel,  A,  B,  is  repre- 
sented as  divided  lengthwise, 
in  order  to  show  the  inside. 
The  piston,  C,  is  fitted  so  ac- 
curately to  the  barrel,  as  to 
work  up  and  down  water-tight ; 
both  ban-el  and  piston  being 
made  of  iron.  The  thing  to 
be  broken  or  pressed,  is  laid 
on  the  flat  surface,  I,  there  be- 
ing above  this,  a  strong  frame 

to  meet  the  pressure,  not  shown  in  the  figure.  The  small 
forcing-pump,  of  which  D  is  the  piston,  and  H,  the  lever  by 
which  it  is  worked,  is  also  made  of  iron. 

Now,  suppose  the  space  between  the  small  piston  and  the 
large  one,  at  W,  to  be  filled  with  water,  then,  on  forcing  down 
the  small  piston,  D,  there  will  be  a  pressure  against  the  large 
piston,  C,  the  whole  force  of  which  will  be  in  proportion  as  the 
aperture  in  which  C  works,  is  greater  than  that  in  which  D 
works. 

436.  If  the  piston,  D,  is  half  an  inch  in  diameter,  and  the 
piston,  C,  one  foot  in  diameter,  then  the  pressure  on  C  will  be  576 
times  greater  than  that  on  D.     Therefore,  if  we  suppose  the 
pressure  of  the  small  piston  to  be  one  ton,  the  large  piston 
will  be  forced  up  against  any  resistance,  with  a  pressure  equal 
to  the  weight  of  576  tons. 

437.  It  would  be  easy  for  a  single  man  to  give  the  pressure 
of  a  ton  at  D,  by  means  of  the  lever,  and,  therefore,  a  man,  with 
this  engine,  would  be  able  to  exert  a  force  equal  to  the  weight 
of  near  600  tons. 

438.  It  is  evident  that  the  force  to  be  obtained  by  this  prin- 
ciple, can  only  be  limited  by  the  strength  of  the  materials  of 
which  the  engine  is  made.     Thus,  if  a  pressure  of  two  tons  be 
given  to  a  piston,  the  diameter  of  which  is  only  a  quarter  of  an 
inch,  the  force  transmitted  to  the  other  piston,  if  three  feet  in 
diameter,  would  be  upward  of  40,000  tons ;  but  such  a  force 


436  In  the  hydrostatic  press,  what  is  the  proportion  between  the  pressure  given 
by  the  small  piston,  and  the  force  exerted  on  the  Jarge  one  1  437.  What  is  the  esti- 
mated force  which  a  man  could  s\\e  by  one  of  these  ensines1?  433.  If  the  pressure 
of  two  tons  be  made  on  a  piston  of  a  quarter  of  an  inch  in  diameter,  what  will  be  the 
force  transmitted  to  the  other  piston  of  three  feet  in  diameter? 


112  PRESSURE    OF   WATER. 

is  much  too  great  for  the  strength  of  any  material  with  which 
we  are  acquainted. 

A  small  quantity  of  water,  extending  to  a  great  elevation, 
would  give  the  pressure  above  described,  it  being  only  for  the 
sake  of  convenience,  that  the  forcing-pump  is  employed  instead 
of  a  column  of  water. 

439.  Rupture  of  a  Mountain. — There  is  no  doubt,  but  in  the 
operations  of  nature,  great  effects  are  sometimes  produced  amor  g 
mountains,  by  a  small  quantity  of  water  finding  its  way  to  a 
reservoir  in  the  crevices  of  the  rocks  far  beneath. 

FIG.  98. 


Rupture  of  a  Mountain. 


Suppose,  in  the  interior  of  a  mountain,  at  A,  Fig.  98,  there 
should  be  a  space  of  ten  yards  square,  and  an  inch  deep,  filled 
with  water,  and  closed  up  on  all  sides  ;  and  suppose  that,  in  the 
course  of  time,  a  small  fissure,  no  more  than  an  inch  in  diam- 
eter, should  be  opened  by  the  water,  from  the  height  of  two 
hundred  feet  above,  down  to  this  little  reservoir.  The  conse- 
quence might  be,  that  the  side  of  the  mountain  would  burst 
asunder,  for  the  pressure,  under  the  circumstances  supposed, 
would  be  equal  to  the  weight  of  five  thousand  tons. 

440.  Pressure  on  Vessels  with  Oblique  Sides. — It  is  obvious, 
that,  in  a  vessel,  the  sides  of  which  are  every  where  perpendic- 
ular to  each  other,  the  pressure  on  the  bottom  will  be  as 
the  height,  and  that  the  pressure  on  the  sides  will  every  where 
be  equal,,  at  an  equal  depth  of  the  liquid. 

But  it  is  not  so  obvious,  that  in  a  vessel  having  oblique  sides, 

439.  What  is  paid  of  the  pressure  of  water  in  the  crevices  of  mountains  and  its 
effects!  440.  What  is  the  pressure  on  the  bottom  of  a  vessel  containing  a  fluid  equal 
to  7  Suppose  the  sides  of  the  vessel  slope  outward,  what  effect  does  this  produce  on 
the  pressure  1 


PRESSURE    OP    WATER.  113 

that  is,  diverging  outward  from  the  bottom,  or  converging  from 
the  bottom  toward  the  top,  in  what  manner  the  pressure  will 
be  sustained. 

441.  Now,  the  pressure  on  the  bottom  of  any  vessel,  no  mat- 
ter what  the  shape  may  be,  is  equal  to  the  height  of  the  fluid, 
and  the  area  of  the  bottom,  (428.) 

Hence  the  pressure  on 

the  bottom  of  the  vessel  FIG.  99. 

sloping  outward,  Fig.  99,  \^^^___-__-f^±----  -  --    --— ^/ 

will  be  just  equal  to  what  ^ f 

it  would  be,  were  the  sides  gjf 

perpendicular,  and  the  same 

Would    be  the  Case  did  the  Pressure  on  Diverging  Sides. 

sides  slope  inward  instead 
cf  outward. 

In  a  vessel  of  this  shape,  the  sides  sustain  a  pressure  equal  to 
the  perpendicular  height  of  the  fluid,  above  any  given  point. 
Thus,  if  the  point  1  sustain  a  pressure  of  one  pound,  2,  being 
twice  as  far  below  the  surface,  will  have  a  pressure  equal  to  two 
pounds,  and  so  in  this  proportion  with  respect  to  the  other  eight 
parts  marked  on  the  side  of  the  vessel.  On  the  contrary,  did 
the  sides  of  the  vessel  slope  inward  instead  of  outward,  still  the 
same  consequences  ensue,  the  vertical  height  in  both  cases  mak- 
ing the  pressure  equal.  For  although  in  the  latter,  the  eleva- 
tion is  not  above  the  point  of  pressure,  the  effect  is  the  same  in 
each  case. 

PRESSURE    OF   WATER   IN   POUNDS,    AT   VARIOUS   DEPTHS. 

442.  The  weight  of  a  cubic  inch  of  water  at  the  temperature 
of  62°,  is  the  0.036065  fraction  of  a  pound.     A  column  of  wa- 
ter one  foot  high,  being  twelve  times  the  above,  would  there- 
fore be  0.4328  pounds. 

443.  Xow  a  square  foot  is  144  square  inches,  and  therefore 
the  pressure,  or  weight,  of  a  square  foot  of  water  will  be  found 
by  multiplying  the  above  fraction  by  144,  which  gives  62.3232. 
?>r  nearly  62  and  a  third  pounds.     Omitting  the  decimals,  a 
cubic  foot  of  water  is  commonly  estimated  at  62  pounds. 

441.  On  the  contrary,  did  the  sides  of  the  vessel  slope  inward  exactly  the  same 
amount  of  pressure  according  to  Ihe  height,  what  would  be  the  result  1  442.  What 
is  the  weight  of  a  cubic  inch  of  water  ?  443.  What  is  the  weight  of  a  cubic  foot  of 
water  7 

10* 


114 


WATER    LEVEL. 


The  following  table,  founded  on  the  above  estimates,  may  be 
useful  in  determining  the  pressure  of  water  in  pipes  or  other 
vessels,  of  known  depth. 


DEPTH    IN    FEET. 

PRESSURE    PER    SQUARE 
INCH. 

PRESSURE    PER    SQUARE 
FOOT. 

• 

Pounds. 

Pounds. 

1 

0.4328 

62.3232 

2 

0.8656 

124.6464 

3 

1.2984 

186,9696 

4 

1.7312 

249.2928 

5 

2.1640 

311.6160 

6 

2.5968 

373.9392 

7 

3.0296 

436.2624 

8 

3.4624 

498.5856 

9 

3,8952 

560.9088 

10 

4.3280 

623.2320 

Suppose  it  is  required  to  know  the  pressure  on  the  bottom 
of  a  vessel  of  water,  1  foot  square  and  20  feet  deep,  then  it  is 
found  by  doubling  that  of  10  feet  deep,  thus  623.2320x2  = 
1246.464  pounds.  The  pressure  on  a  tube  equal  to  an  inch 
square,  and  of  an  equal  depth,  is  found  by  substituting  inches  for 
feet,  as  above  seen. 


WATER   LEVEL. 


444.  We  have  seen,  that   in  whatever   situation   water   is 
placed,  it  always  tends  to  seek  a  level.     Thus,  if  several  vessels 
communicating  with  each  other  be  filled  with  water,  the  fluid 
will  be  at  the  same  height  in  all,  and  the  level  will  be  indica- 
ted by  a  straight  line  drawn  through  all  the  vessels  as  in  Fig.  91. 

It  is  on  the  principle  of  this  tendency  that  the  little  instru- 
ment called  the  water  level  is  constructed. 

445.  Let  A,  Fig.  100,  be  a  straight  glass  tube  having  two 
legs,  or  two  other  glass  tubes  rising  from  each  end  at  right- 
angles.     Let  the  tube  A,  and  a  part  of  the  legs,  be  filled  with 
mercury  or  some  other  liquid,  and  on  the  surfaces,  a  6,  of  the 
liquid,  let  floats  be  placed,  carrying  upright  wires,  to  the  ends 
of  which  are  attached  sights  at  1,  2.     These  sights  are  repre- 
sented by  3,  4,  and  consist  of  two  fine  threads,  or  hairs,  stretched 


445.  Explain  by  Fig.  100,  how  an  exact  line  may  be  obtained  by  adjusting  th« 
lights. 


WATER   LEVEL. 
FIG.  100. 


Improved  Water  Level. 

at  right-angles  across  a  square,  and  are  placed  at  right-angles  to 
the  length  of  the  instrument. 

They  are  so  adjusted  that  the  point  where  the  hairs  intersect 
each  other,  shall  be  at  equal  heights  abore  the  floats.  This  ad- 
justment may  be  made  in  the  following  manner : — 

Let  the  eye  be  placed  behind  one  of  the  sights,  looking 
through  it  at  the  other,  so  as  to  make  the  points,  where  the 
hairs  intersect,  cover  each  other,  and  let  some  distant  object, 
covered  by  this  point,  be  observed.  Then  let  the  instrument 
be  reversed,  and  let  the  points  of  intersection  of  the  hairs  be 
viewed  in  the  same  way,  so  as  to  cover  each  other.  If  they  are 
observed  to  cover  the  same  distant  object  as  before,  they  will 
be  of  equal  heights  above  the  surfaces  of  the  liquid.  But,  if  the 
same  distant  points  be  not  observed  in  the  direction  of  these 
points,  then  one  or  the  other  of  the  sights  must  be  raised  or 
lowered,  by  an  adjustment  provided  for  that  purpose,  until  the 
points  of  intersection  be  brought  to  correspond.  The  points 
will  then  be  properly  adjusted,  and  the  line  passing  through 
them  will  be  exactly  "horizontal.  All  points  seen  in  the  direc- 
tion of  the  sights  will  be  on  the  level  of  the  instrument. 

44fi.  The  principles  on  which  this  adjustment  depends  are 
easily  explained  :  if  the  intersections  of  the  hairs  be  at  the  same 
distance  from  the  floats,  the  line  joining  these  intersections  will 
evidently  be  parallel  to  the  lines  joining  the  surfaces  a,  6,  of  the 

446.  Explain  the  principle  on  which  the  water  level  with  sights  is  constructed. 


116  SPECIFIC    GRAVITY. 

liquid,  and  will  therefore  be  level.  But  if  one  of  these  points 
be  more  distant  from  the  floats  than  the  other,  the  line  joining 
the  intersections  will  point  upward  if  viewed  from  the  lower 
sight,  and  downward  if  viewed  from  the  higher  one. 

The  accuracy  of  the  results  of  this  instrument,  will  be  greatly 
increased  by  lengthening  the  tube  A. 

447.  Spirit  Level— The  common  ™G  m-  • 
spirit  level  consists  of  a  glass  tube,              a 

Fig.  101,  filled  with  spirit  of  wine,  ex-       <*}~"  }*» 

cepting  a  small  space  in  which  there 

is  left  a  bubble  of  air.     This  bubble,  spirit  Level. 

when  the  instrument  is  laid  on  a  level 

surface,  will  be  exactly  in  the  middle  of  the  tube,  and  therefore, 

to  adjust  a  level,  it  is  only  necessary  to  bring  the  bubble  to  this 

position. 

The  glass  tube  is  inclosed  in  a  brass  case,  which  is  cut  out 
on  the  upper  side,  so  that  the  bubble  may  be  seen,  as  repre- 
sented in  the  figure. 

448.  This  instrument  is  employed  by  builders  to  level  their 
work,  and  is  highly  convenient  for  that  purpose,  since  it  is  only 
necessary  to  -lay  it  on  a  beam  to  try  its  level. 


SPECIFIC    GRAVITY. 


449.  If  a  tumbler  be  filled  with  water  to  the  brim,  and  an 
egg,  or  any  other  heavy  solid,  be  dropped  into  it,  a  quantity  of 
the  jluid,  exactly  equal  to  the  size  of  the  egg,  or  other  solid,  will 
be  displaced,  and  will  flow  over  the  side  of  the  vessel.     Bodies 
which  sink  in  water,  therefore,  displace  a  quantity  of  the  fluid 
equal  to  their  own  bulks. 

450.  Now,  it  is  found  by  experiment,  that  when  any  solid 
substance  sinks  in  water,  it  loses,  while  in  the  fluid,  a  portion  of 
its  weight,  just  equal  to  that  of  the  bulk  of  water  which  it  dis- 
places.    This  is  readily  made  evident  by  experiment. 

451.  Take  a  piece  of  ivory,  or  any  other  substance  that  will 
sink  in  water,  and  weigh  it  accurately  in  the  usual  manner ; 
then  suspend  it  by  a  thread,  or  hair,  in  the  empty  cup  A,  Fig. 
102,  and  balance  it,  as  shown  in  the  figure.     Now  pour  water 
into  the  cup,  and  it  will  be  found  that  the  suspended  body  will 
lose  a  part  of  its  weight,  so  that  a  certain  number  of  grains 

447.  Describe  the  common  spirit  level,  and  the  method  of  using  it.  448.  What  is 
the  use  of  the  level  7  449.  How  much  water  will  an  eg*  displace?  450.  How  much 
less  will  a  cubic  inch  of  any  substance  weigh  in  water  than  in  air  ]  451.  How  is  it 
proved  by  Fig.  102,  that  a  body  weighs  less  in  water  than  in  air  1 


SPECIFIC    GRAVITY. 


117 


Weighing  in  Water. 


must  be  taken  from  the 
opposite  scale,  in  order  to 
make  the  scales  balance  as 
before  the  water  was  pour- 
ed in.  The  number  of 
grains  taken  from  the  op- 
posite scale,  show  the 
weight  of  a  quantity  of 
water  equal  to  the  bulk  of 
the  body  so  suspended. 

452.  It  is  on  the  prin- 
ciple, that  bodies  weigh  less 
in  the  water  than  they  do 
when  weighed  out  of  it,  or 
in  the  air,  that  water  be- 
comes the  means  of  ascertaining  their  specific  gravities,  for  it  is 
by  comparing  the  weight  of  a  body  in  the  water,  with  what  it 
weighs  out  of  it,  that  its  specific  gravity  is  determined. 

Thus,  suppose  a  cutfic  inch  of  gold  weighs  19  ounces,  and  on 
being  weighed  in  water,  weighs  only  18  ounces,  or  loses  a  nine- 
teenth part  of  its  weight,  it  will  prove  that  gold,  bulk  for  bulk, 
is  nineteen  times  heavier  than  water,  and  thus  19  would  be  the 
specific  gravity  of  gold.  And  so  if  a  cube  of  copper  weigh  9 
ounces  in  the  air,  and  only  8  ounces  in  the  water,  then  copper, 
bulk  for  bulk,  is  9  times  as  heavy  as  water,  and  therefore  has  a 
specific  gravity  of  9. 

453.  If  the  body  weighs  less,  bulk  for  bulk,  than  water,  it  is 
obvious  that  it  will  not  sink  in  it,  and  therefore  weights  must 
be  added  to  the  lighter  body,  to  ascertain  how  much  less  it 
weighs  than  water. 

The  specific  gravity  of  a  body,  then,  is  merely  its  weight 
compared  with  the  same  bulk  of  water ;  and  water  is  thus  made 
the  standard  by  which  the  weights  of  all  other  bodies  are 
compared. 

454.  How  to  take  the  Specific  Gravity.— To  take  the  specific 
gravity  of  a  solid  which  sinks  in  water,  first  weigh  the  body  in 
the  usual  manner,  and  note  down  the  number  of  grains  it 
weighs;  then,  with  a  hair,  or  fine  thread,  suspend  it  from  the 
bottom  of  the  scale-dish,  in  a  vessel  of  water,  as  represented  by 
Fig.  102.     As  it  weighs  less  in  water,  weights  must  be  added 
to  the  side  of  the  scale  where  the  body  is  suspended,  until  they 


452.  On  what  principle  are  specific  gravities  found?    453.  What  is  the  specific 
gravity  of  a  body  7    454.  How  are  the  specific  gravities  of  solid  bodies  taken  ? 


118 


SPECIFIC    GRAVITY. 


exactly  balance  each  other.  Next,  note  down  the  number  of 
grains  so  added,  and  they  will  show  the  difference  between  the 
weight  of  the  body  in  air  and  in  water. 

455.  It  is  obvious  that  the  greater  the  specific  gravity  of  the 
body,  the  less,  comparatively,  will  be  this  difference,  because 
each  body  displaces  only  its  own  bulk  of  water,  and  some  bodies 
of  the  same  bulk  will  weigh  many  times  more  than  others. 

456.  For  example,  suppose  that  a  piece  of  platina,  weighing 
22  ounces,  will  displace  an  ounce  of  water,  while  a  piece  of 
silver,  weighing  22  ounces,  will  displace  two  ounces  of  water. 
The  platina,  therefore,  when  suspended  as  above  described,  will 
require  one  ounce  to  make  the  scales  balance,  while  the  same 
weight  of  silver  will  require  two  ounces  for  the  same  purpose. 
The  platina,  therefore,  bulk  for  bulk,  will  weigh  twice  as  much 
as  the  silver,  and  will  have  twice  as  much  specific  gravity. 

Having  noted  down  the  difference  between  the  weight  of  the 
body  in  air  and  in  water,  as  above  explained,  the  specific  gravity 
is  found  by  dividing  the  weight  in  air  by  the  loss  in  water. 
The  greater  the  loss,  therefore,  the  less  will  be  the  specific 
gravity,  the  bulk  being  the  same. 

457.  Thus,  in  the  above  example,  22  ounces  of  platina  was 
supposed  to  lose  one  ounce  in  water,  while  22  ounces  of  silver 
lost  two  ounces  in  water.     Now,  22  divided  by  1,  the  loss  of 
the  platina,  is  22 ;  and  22  divided  by  2,  the  loss  in  the  silver, 
is  11.     So  that  the  specific  gravity  of  platina  is  22,  while  that 
of  silver  is  11.     The  specific  gravities  of  these  metals  are,  how- 
ever, a  little  less  than  here  estimated. 

458.    TABLE    OF   SPECIFIC    GRAVITIES. 


Antimony, 7 

Zinc, 7 

Cast  Iron, 7 

Tin, 8 

Cobalt, 8 

Steel, 8 

Copper, 9 

Bismuth, 10 

Silver, 10 

Lead, 11 

Gold, 19 


Platinum, 20 

"         hammered,      ...  22 

Mercury, 14 

Agate, 2* 

Sulphur, 2 

Glass,  crown, 2$ 

"        flint, 3i 

Rock,  crystal, 24 

Marble, 24 

Diamonds, 3i 

Ruby,  (oriental,) 4i 


This  table  being  intended  for  common  use,  the  fractions  are 
omitted,  and  the  nearest  round  numbers  only  given. 

455  Why  does  a  heavy  body  weigh  comparatively  less  in  the  water  than  a  lijrht 
one?  456  Having  taken  the  difference  between  the  weight  of  a  body  in  air  and  in 
water,  by  what  rule  is  its  specific  gravity  found?  457.  Give  the  example  stated,  and 
show  how  the  difference  between  the  specific  gravities  of  platina  and  silver  is  found- 


HYDROMETER.  119 


HYDROMETER. 

459.  The  hydrometer  is  an  instrument  by  which  the  specific 
gravities  of  fluids  are  ascertained  by  the  depth  to  which  the  in- 
strument sinks  below  their  surfaces. 

460.  Suppose  a  cubic  inch  of  lead  loses,  when  weighed  in 
water,   253  grains,  and,  when  weighed  in  alcohol,   only  209 
grains,  then,  according  to  the  principle  already  recited,  a  cubic 
inch  of  water  actually  weighs  253,  and  a  cubic  inch  of  alcohol 
209  grains,  for  when  a  body  is  weighed  in  a  fluid,  it  loses  just 
the  weight  of  the  fluid  it  displaces. 

461.  Water,  as  we  have  already  seen,  (453,)  is  the  standard 
by  which  the  weights  of  other  bodies  are  compared,  and  by 
ascertaining  what  a  given  bulk  of  any  substance  weighs  in  wa- 
ter, and  then  what  it  weighs  in  any  other  fluid,  the  compara- 
tive weight  of  water  and  this-  fluid  will  be  known.     For  if,  as  in 
the  above  example,  a  certain  bulk  of  water  weighs  253  grains, 
and  the  same  bulk  of  alcohol  only  209  grains,  then  alcohol  has 
a  specific  gravity  nearly  one-fourth  less  than  water. 

462.  It  is  on  this  principle  that  the  hydrom- 
eter is  constructed.     It  is  composed  of  a  hoi- 
low  ball  of  glass,  or  metal,  with  a  graduated 
scale  rising  from  its  upper  part,  and  a  weight 
on  its  under  part,  which  serves  to  balance  it  in 
the  fluid. 

Such  an  instrument  is  represented  by  Fig. 
103,  of  which  B  is  the  graduated  scale,  and  A, 
the  weight,  the  hollow  ball  being  between 
them. 

463.  To  prepare   this  instrument   for   use, 
weights  in  grains,  or  half-grains,  are  put  into 
the  little  cup,  A,  until  the  scale  is  carried  down 
so  that  a  certain  mark  on  it  coincides  exactly 
with    the   surface  of  the  water.     This  mark, 

then,  becomes  the  standard  of  comparison  be-        Hydrometer. 
tween  water  and  any  other  liquid  in  which  the 
hydrometer  is  placed. 

464.  If  plunged  into  a  fluid  lighter  than  water,  it  will  sink 
below  the  mark,  and,  consequently,  the  fluid  will  rise  higher  on 


459.  What  is  the  hydrometer  ?  460.  Suppose  a  cubic  inch  of  any  substance  weighs 
253  grains  less  in  water  than  in  air.  what  is  the  actual  weight  of  a"  cubic  inch  of  wa- 
ter! 461.  On  what  principle  is  the  hydrometer  founded  ?  462.  How  is  this  instru- 
ment formed  ?  463.  How  is  the  hydrometer  prepared  for  use  ?  464.  How  is  it 
known  by  this  instrument  whether  the  lluid  is  lighter  or  heavier  than  wafer  7 


120  SIPHON. 

the  scale.  If  the  fluid  is  heavier  than  water,  the  scale  will  rise 
above  the  surface  in  proportion,  and  thus  it  is  ascertained  in  a 
moment  whether  any  fluid  has  a  greater  or  less  specific  gravity 
than  water. 

To  know  precisely  how  much  the  fluid  varies  from  the 
standard,  the  scale  is  marked  off  into  degrees,  which  indicate 
grains  by  weightj  so  that  it  is  ascertained  very  exactly  how  much 
the  specific  gravity  of  one  fluid  differs  from  that  of  another. 

465.  Water  being  the  standard  by  which  the  weights  of 
other  substances  are  compared,  it  is  placed  as  the  unit,  or  point 
of    comparison,  and  is,  therefore,   1, '10,   100,   or    1000,  the 
ciphers  being  added  whenever  there  are  fractional  parts  ex- 
pressing the  specific  gravity  of  the  body.     It  is  always  under- 
stood, therefore,  that  the  specific  gravity  of  water  is  1 ;  and 
when  it  is  said  a  body  has  a  specific  gravity  of  2,  it  is  only 
meant  that  such  a  body  is,  bulk  for  bulk,  twice  as  heavy  as 
water. 

466.  If  the  substance  is  lighter  than  water,  it  has  a  specific 
gravity  of  0,  with  a  fractional  part.     Thus,  alcohol  has  a  specific 
gravity  of  0.809,  that  is,  809,  water  being  1000. 

467.  By  means  of  this  instrument,  it  can  be  told  with  great 
accuracy  how  much  water  has  been  added  to  spirits,  for  the 
greater  the  quantity  of  water,  the  higher  will  the  scale  rise 
above  the  surface. 

The  adulteration  of  milk  with  water,  can  also  be  readily  de- 
tected with  it,  for  as  new  milk  has  a  specific  gravity  of  1032, 
water  being  1000,  a  very  small  quantity  of  water  mixed  with 
it  would  be  indicated  by  the  instrument. 

THE    SIPHON. 

468.  Take  a  tube  bent  like  the  letter  U,  and,  having  filled  it 
with  water,  place  a  finger  on  each  end,  and  in  this  state  plunge 
one  of  the  ends  into  a  vessel  of 'water,  so  that  the  end  in  the 
water  shall  be  a  little  the  highest ;  then  remove  the  fingers  and 
the  liquid  will  flow  out,  and  continue  to  do  so  until  the  vessel 
is  exhausted. 

469.  A  tube  acting  in  this  manner  is  called  a  siphon,  and  is 

465.  What  is  the  standard  by  which  the  weights  of  other  bodies  are  compared  ? 
What  is  the  specific  gravity  of  water?  When  it  is  said  that  the  specific  gravity  of  a 
borly  is  2,  or 4,  what  meaning  is  intended  to  be  conveyed?  466.  If  alcohol  has  a 
specific  gravity  of  809;  what,  in  reference  to  this,  is  the  specific  gravity  of  water? 
467.  In  what  cases  will  the  hydrometer  detect  fraud  ?  468.  In  what  manner  is  a 
Biphon  made  1  469.  Explain  the  reason  why  the  water  ascends  through  one  leg  of 
.he  siphon,  and  descends  through  the  other  1 


BIPHON. 


121 


represented  by  Fig.  104.     The  reason  FIG.  lot 

why  the  water  flows  from  the  end  of 

the  tube,  A,  and,  consequently,  ascends 

through  the  other  part,  is,  that  there 

is  a  greater  weight  of  the  fluid  from  B 

to  A,  than  from  C  to  B,  because  the 

perpendicular  height  from  B  to  A,  is 

the  greatest.     The  weight  of  the  water 

from  B  to  A,  falling  downward  by  its 

gravity,  tends  to  form   a  vacuum,  or 

void  space,  in  that  leg  of  the  tube ;  but 

the  pressure  of  the  atmosphere  on  the 

water  in  the  vessel,  constantly  forces  Siphon. 

the  fluid  up  the  other  leg  of  the  tube, 

to  fill  the  void  space,  and  thus  the  stream  is  continued  as  long 

as  any  water  remains  in  the  vessel. 

470.  Ajjplicatian  of  the  Siphon.- — The  siphon  is  employed 
in  draining  mines,  when  there  is  a  sufficient  fall  in  the  vicinity : 
it  may  also  be  used  to  convey  water  over  a  hill,  provided  the 
place  where  it  is  wanted  is  a  foot  or  two  lower  than  the  fountain. 


Application  of  the  Siphon. 

For  this  purpose,  let  A  be  a  spring,  Fig.  105,  situated  be 
hind  a  hill,  and  it  is  desired  to  bring  the  water  to  B  for  family 
use.  To  do  this,  a  lead  tube,  with  a  stop-cock  at  C,  is  carried 
over  the  hill,  having  also  a  stop-cock  at  each  end.  This  done, 
and  the  two  ends  being  closed,  fill  the  two  legs  of  the  tube  by 
pouring  in  water  at  C  ;  then  C  being  closed,  let  one  person  open 
the  stop-cock  at  B,  and  a  moment  after,  open  that  at  A,  and 
the  water  will  instantly  begin  to  flow  from  the  spring  to  the 
reservoir,  and  if  C  is  kept  closed,  will  continue  to  run  so  long 
as  the  fountain  furnishes  water. 


470.  Explain  by  Fig.  105,  how  the  siphon  o/»oveys  water  over  a  hilL 

6 


122 


INTERMITTING    SPRINGS. 


The   principle   of   the   siphon  has   been    explained  under 
Fig.  104. 


INTERMITTING    SPRINGS. 


471.  The  action  of  the  siphon  depends  upon  the  same  prin- 
ciple as  the  action  of  the  pump,  namely,  the  pressure  of  the 
atmosphere,  and,  therefore,  its  explanation  properly  belongs  to 
Pneumatics.  It  is  introduced  here  merely  for  the  purpose  of 
illustrating  the  phenomena  of  intermitting  springs,  a  subject 
which  belongs  to  Hydrostatics. 

Some  springs,  situated  on  the  sides  of  mountains,  flow,  for  a 
while,  with  great  violence,  and  then  cease  entirely.  After  a 
time  they  begin  to  flow  again,  and  then  suddenly  stop,  as  be- 
fore. These  are  called  intermitting  springs.  Among  ignorant 
and  superstitious  people,  these  strange  appearances  have  been 
attributed  to  witchcraft,  or  the  influence  of  some  supernatural 
power.  But  an  acquaintance  with  the  laws  of  nature  will  dis- 
sipate such  ill-founded  opinions,  by  showing  that  they  owe  their 
peculiarities  to  nothing  more  than  natural  siphons,  existing  in 
the  mountains  from  whence  the  water  flows. 

FIG.  106. 


Intermitting  Spring. 


472.  Fig.  106  is  the  section  of  a  mountain  and  spring,  show- 
ing how  the  principle  of  the  siphon  operates  to  produce  the 
effect  described.  Suppose  there  is  a  crevice,  or  hollow,  in  the 
rock  from  A  to  B,  and  a  narrow  fissure  leading  from  it,  in 
the  form  of  the  siphon,  B  C.  The  water  from  the  rills  F  E, 


471.  What  is  an  intermitting  spring  1    472,  How  is  the  phenomenon  of  the  inter 
mitting  spring  explained  1 


HYDRAULICS.  123 

filling  the  hollow,  up  to  the  line  A  D,  it  will  then  discharge 
itself  through  the  siphon,  and  continue  to  run  until  the  water 
is  exhausted  down  to  the  leg  of  the  siphon  B,  when  it  will 
cease.  Then  the  water  from  the  rills  continuing  to  run  until 
the  hollow  is  again  filled  up  to  the  same  line,  the  siphon  again 
begins  to  act,  and  again  discharges  the  contents  of  the  reservoir 
as  before,  and  thus  the  spring  P,  at  oi*e  moment  flows  with 
great  violence  and  the  next  moment  ceases  entirely. 

473.  The  hollow,  above  the  line  A  D,  is  supposed  not  to  be 
filled  with  the  water  at  all,  since  the  siphon  begins  to  act  when- 
ever the  fluid  rises  up  to  the  bend  D. 

During  the  dry  seasons  of  the  year,  it  is  obvious,  that  such  a 
spring  would  cease  to  flow  entirely,  and  would  begin  again  only 
when  the  water  from  the  mountain  filled  the  cavity  through 
the  rills. 


CHAPTER  VI. 

HYDRAULICS. 

474.  It  has  been  stated,  (410,)   thai  Hydrostatics  is  that 
branch  of  Natural  Philosophy,  which  treats  of  the  weight,  pres- 
sure, and  'equilibrium  of  fluids,  and  that  Hydraulics  has  for 
its  object,  the  investigation  of  the  laws  which  regulate  fluids  in 
motion. 

If  the  pupil  has  learned  the  principles  on  which  the  pres- 
sure and  equilibrium  of  fluids  depend,  as  explained  under  the 
former  article,  he  will  now  be  prepared  to  understand  the  laws 
which  govern  fluids  when  in  motion. 

475.  The  pressure  of  water  downward,  is  in  the  same  pro- 
portion to  its  height,  as  is  the  pressure  of  solids  in  the  same 
direction. 

476.  Suppose  a  vessel  of  three  inches  in  diameter  has  a  billet 
of  wood  set  up  in  it,  so  as  to  touch  only  the  bottom,  and  sup- 
pose the  piece  of  wood  to  be  three  feet  long,  and  to  weigh  nine 
pounds ;  then  the  pressure  on  the  bottom  of  the  vessel  will  be 

472.  Explain  Fi2.  106,  and  show  the  reason  why  such  a  spring  will  flow  and  cease 
lo  flow,  alternately.  474.  How  does  the  science  of  Hydrostatics  differ  from  that  of 
Hydraulics  ?  475.  Does  the  downward  pressure  of  water  differ  from  the  downward 
pressure  of  solids,  in  proportion  1  476.  How  is  the  downward  pressure  of  water 
illustrated  1 


124 


HYDRAULICS. 


nine  pounds.  If  another  billet  of  wood  be  set  on  this,  of  the 
same  dimensions,  it  will  press  on  its  top  with  the  weight  of 
nine  pounds,  and  the  pressure  at  the  bottom  will  be  eighteen 
pounds,  and  if  a  like  billet  be  set  on  this,  the  pressure  at  the 
bottom  will  be  twenty-seven  pounds,  and  so  on,  in  this  ratio,  to 
any  height  the  column  is  carried. 

Now  the  pressure  cf  fluids  is  in  the  same  proportion;  and 
when  confined  in  pipes,  may  be  considered  as  one  short  column 
set  on  another,  each  of  which  increases  the  pressure  of  the 
lowest,  in  proportion  to  their  number  and  height. 

477.  If  a  vessel, 

Fig.   107,  be  filled  FIG-  107 

with  water,and  three 
apertures  be  made 
in  its  side  at  E  F  G, 
the  fluid  will  be 
thrown  out  in  jets, 
falling  to  the  earth 
in  the  curved  lines 
shown.  The  reason 
why  these  curves 
differ  in  shape,  is, 
that  the  fluid  is  act- 
ed on  by  two  forces, 
namely,-  the  pres- 
sure of  the  water 
above  the  jet,  which  produces  its  velocity  forward,  and  the  ac- 
tion of  gravity,  which  impels  it  downward.  It  therefore  obeys 
the  same  laws  that  solids  do  when  projected  forward,  and  falls 
down  in  curved  lines,  the  shapes  of  which  depend  on  their  rela- 
tive velocities,  (246.) 

478.  The  quantity  of  water  discharged,  being  in  proportion 
to  the  pressure,  when  the  orifices  are  the  same,  that  discharged 
from  each  orifice  will  differ  in  quantity,  according  to  the  height 
of  the  water  above  it. 

479.  It  is  found,  however,  that  the  velocity  with  which  a 
vessel  discharges  its  contents,  does  not  depend  entirely  on  the 
pressure,  but  in  part  on  the  kind  of  orifice  through  which  the 
liquid  flows.     It  might  be  expected,  for  instance,  that  a  tin  v^es- 


Velocity  and  Gravity. 


477.  Why  do  the  lines  described  by  the  jets  from  the  vessel,  Fig.  107, -differ  in 
shape?  What  two  forces  act  upon  the  fluid  as  it  is  discharged,  and  how  do  these 
forces  produce  a  curved  line?  478.  In  what  proportion  do  the  orifices  discharge  the 
fluid  7  479.  Does  th«  velocity  with  which  a  lluid  is  discharged,  depend  entirely  on 
thepressuro? 


HYDRAULICS.  125 

sel  of  a  gwen  capacity,  with  an  orifice  of,  say  an  inch  in  diam- 
eter, would  part  with  its  contents^  sooner  than  another  of  the 
same  capacity  and  orifice,  whose  side  was  an  inch  or  two  thick, 
since  the  friction  through  the  tin  might  be  considered  much 
less  than  that  presented  by  the  other  orifice. 

480.  But  it  has  been  found,  by  experiment,  that  the  tin  ves- 
sel does  not  part  with  its  contents  so  soon  as  another  vessel,  of 
the  sarne  height  and  size  of  orifice,  from  which  the  water  flowed 
through  a  short  pipe.     And,  on  varying  the  length  of  these 
pipes,  it  is  found  that  the  most  rapid  discharge,  other  circum- 
stances being  equal,  is  through  a  pipe,  whose  length  is  twice  the 
diameter  of  its  orifice.     Such  an  aperture  discharged  82  quarts, 
in  the  same  time  that  another  vessel  of  tin,  without  the  pipe, 
discharged  62  quarts. 

481.  This  surprising  difference  is  accounted  for,  by  supposing 
that  the  cross  currents,  made  by  the  rushing  of  the  water  from 
different  directions  toward  the  orifice,  mutually  interfere  with 
each  other,  by  which  the  whole  is  broken,  and  thrown  into  con- 
fusion by  the  sharp  edge  of  the  tin,  and  hence  the  water  issues 
in  the  form  of  spray,  or  of  a  screw,  from  such  an  orifice.     A 
short  pipe  seems  to  correct  this  contention  among  opposing 
currents,  and  to  smooth  the  passage  of  the  whole,  and  hence 
we  may  observe,  that  from  such  a  pipe,  the  stream  is  round  and 
well  defined. 

482.  Proportion  between  the  Pressure  and  the  Velocity  of 
Discharge. — If  a  small  orifice  be  made  in  the  side  of  a  vessel 
filled  with  any  liquid,  the  liquid  will  flow  out  with  a  force  and 
velocity  equal  to  the  pressure  which  the  liquid  before  exerted 
on  that  portion  of  the  side  of  the  vessel  before  the  orifice  was 
made. 

Now,  as  the  pressure  of  fluids  is  as  their  heights,  it  follows, 
that  if  several  such  orifices  are  made,  the  lowest  will  discharge 
the  greatest,  while  the  highest  will  discharge  the  least  quantity 
of  the  fluid. 

The  velocity  of  discharge,  in  the  several  orifices  of  such  a 
vessel,  will  show  a  remarkable  coincidence  between  the  ratio 
of  increase  in  the  quantity  of  liquid,  and  the  increased  ve- 
locity of  a  falling  body. 


480.  What  circumstance,  besides  pressure,  facilitates  the  discharge  of  water  from 
an  orifice  ?  In  a  tube  discharging  water  with  the  greatest  velocity,  what  is  the  pro- 
portion between  its  diameter  and  its  length  ]  What  is  the  proportion  between  the 
quantity  of  fluid  discharged  through  an  orifice  of  tin  and  through  a  short  pipe  ?  481. 
How  is  this  difference  explained  }  482.  What  are  the  proportions  between  the  ve- 
locities of  discharge  and  the  heights  of  the  orifices,  as  above  explained  7 


126 


HYDRAULICS. 


483.  Thus,  if  the  tall  vessel,  Fig.  108, 
of  equal  dimensions  throughout,  be  filled 
with   the  water,  and  a  small  orifice  be 
made  at  one  inch  from  the  top,  or  below 
the  surface,  as  at  1 ;  and  another  at  2,  4 
inches  below  this ;  another  at  9  inches ; 
a  fourth  at  16  inches ;  and  a  fifth  at  25 
inches ;  then  the  velocities  of  discharge, 
from  these  several  orifices,  will  be  in  pro- 
portion of  1,  2,  3,  4,  5. 

484.  To  make  this  more  obvious,  we 
will  place  the  expressions  of  the  several 
velocities  in  the  upper  line  of  the  following 
table,  the  lower  numbers  expressing  the 
depths  of  the  several  orifices. 


FIG.  108. 


-i 


Velocity  of  Discharge. 


Velocity,.  ... 
Depth,  ..... 


41    51    61    71    81    91    10 
16     25     36     49     64     81     100 


Thus  it  appears,  as  in  falling  bodies,  that  to  produce  a  two- 
fold velocity  a  fourfold  height  is  necessary.  To  obtain  a  three- 
fold velocity  of  discharge,  a  ninefold  height  is  required,  and  for 
a  fourfold  velocity,  sixteen  times  the  height,  and  so  in  this  pro- 
portion, as  shown  by  the  table,  (111.) 

In  order  to  establish  the  fact,  that  the  velocity  with  which  a 
liquid  spouts  from  an  orifice,  is  equal  to  the  velocity  which 
a  body  would  acquire  in  falling  unobstructed  from  the^surface 
of  the  liquid  to  the  depth  of  the  orifice,  it  is  only  necessary  to 
prove  the  truth  of  the  principle  in  any  one  particular  case. 

Now  it  is  manifestly  true,  if  the  orifice  be  presented  down- 
ward, and  the  column  of  fluid  over  it  be  of  small  height,  then 
this  indefinitely  small  column  will  drop  out  of  the  orifice  by  the 
mere  effect  of  its  own  weight,  and,  therefore,  with  the  same 
velocity  as  any  other  falling  body ;  but  as  fluids  transmit  pres- 
sure in  all  directions,  the  same  effect  will  be  produced,  whatever 
may  be  the  direction  of  the  orifice. 

FRICTION   BETWEEN    SOLIDS   AND    FLUIDS. 

485.  The  rapidity  with  which  water  flows  through  pipes  of 
the  same  diameter,  is  found  to  depend  much  on  the  nature 

483.  If  in  Fig.  108.  orifices  are  made  at  the  distance  of  1.4,  9, 16,  and  25  inches  from 
the  top,  then  in  what  ratio  of  velocity  will  the  water  be  discharged  1  484.  How  is  it 
proved  that  the  velocity  of  the  spouting  liquid  is  equal  to  that  of  a  falling  body  ?  485. 
Suppose  a  lead  and  a  glass  tube,  of  the  same  diameter  ;  which  will  deliver  the  greatest 
quantity  of  liquid  in  the  same  time  7  Why  will  a  glass  tube  deliver  most  7 


SOLIDS    AND    FLUIDS.  127 

of  their  internal  surfaces.  Thus  a  lead  pipe,  with  a  smooth 
aperture,  under  the  same  circumstances,  will  convey  much  more 
water  than  one  of  wood,  where  the  surface  is  rough,  or  beset 
with  points.  In  pipes,  even  where  the  surface  is  as  smooth  as 
glass,  there  is  still  considerable  friction,  for  in  all  cases,  the  wa- 
ter is  found  to  pass  more  rapidly  in  the  middle^f  the  stream 
than  it  does  on  the  outside,  where  it  rubs  against  the  sides  of 
the  tube. 

486.  The  sudden  turns,  or  angles  of  a  pipe,  are  also  found  to 
be  a  considerable  obstacle  to  the  rapid  conveyance  of  the  water, 
for  such  angles  throw  the  fluid  into  eddies  or  currents  by  which 
its  velocity  is  arrested. 

In  practice,  therefore,  sudden  turns  are  generally  avoided,  and 
where  it  is  necessary  that  the  pipe  should  change  its  direction, 
it  is  done  by  means  of  as  large  a  circle  as  convenient. 

48*7.  Water  in  Pipes. — Where  it  is  proposed  to  convey  a 
certain  quantity  of  water  to  a  considerable  distance  in  pipes, 
there  will  be  a  great  disappointment  in  respect  to  the  quantity 
actually  delivered,  unless  the  engineer  takes  into  account  the 
friction,  and  the  turnings  of  the  pipes,  and  makes  large  allow- 
ances for  these  circumstances.  K  the  quantity  to  be  actually 
delivered  ought  to  fill  a  two-inch  pipe,  one  of  three  inches  will 
not  be  too  great  an  allowance,  if  the  water  is  to  be  conveyed  to 
any  considerable  distance. 

In  practice,  it  will  be  found  that  a  pipe  of  two  inches  in  diam- 
eter, one  hundred  feet  long,  will  discharge  about  five  times  as 
much  water  as  one  of  one  inch  in  diameter  of  the  same  length, 
and  under  the  same  pressure. 

488.  This  difference  is  accounted  for,  by  supposing  that  both 
tubes  retard  the  motion  of  the  fluid,  by  friction,  at  equal  dis- 
tances from  their  inner  surfaces,  and  consequently  that  the  effect 
of  this  cause  is  much  greater  in  proportion,  in  a  small  tube, 
than  in  a  large  one. 

489.  flowing  of  Rivers. — The  effect  of  friction  in  retarding 
the  motion  "of  fluids  is  perpetually  illustrated  in  the  flowing  of 
rivers  and  brooks.     On  the  side  of  a  river,  the  water,  especially 
where  it  is  shallow,  is  nearly  still,  while  in  the  middle  of  a 
stream  it  may  run  at  the  rate  of  five  or  six  miles  an  hour.     For 
the  same  reason,  the  water  at  the  bottoms  of  rivers  is  much  less 


486.  What  is  said  of  the  sudden  turnings  of  a  tube,  in  retarding  the  motion  of  the 
fluid?  487.  How  much  more  water  will  a  two-inch  tube  of  a  hundred  feet  long  dis- 
charge, than  a  one-inch  tube  of  the  same  length  ?  488.  How  is  this  difference  ac- 
counted for  ?  489.  How  do  rivers  show  the  effect  of  friction  in  retarding  the  motion 
of  their  waters? 


128  RAISING    WATER. 

rapid  than  at  the  surface.  This  is  often  proved  by  the  oblique 
position  of  floating  substances,  which  in  still  water  would  assume 
a  vertical  direction. 

Thus,  suppose  the  stick  of  wrood  E,  FIG-  109- 

Fiff.  109,  to  be  loaded  at  one  end  with 
lead,  of  the  sSne  diameter  as  the  wood, 
so  as  to  make  it  stand  upright  in  still 
water.  In  the  current  of  a  river,  where 
the  lower  end  nearly  reaches  the  bottom, 
it  will  incline  as  in  the  figure,  because 
the  water  is  more  rapid  toward  the 
surface  than  at  the  bottom,  and  hence 
the  tendency  of  the  upper  end  to  move 
faster  than  the  lower  one,  gives  it  an 
inclination  forward.  River— Current. 

MACHINES   FOR    RAISING    WATER. 

490.  The  common  pump,  though  a  hydraulic  machine,  de- 
pends on  the  pressure  of  the  atmosphere  for  its  effect,  and  there- 
fore its  explanation  comes  properly  under  the  article  Pneumatics, 
where  the  consequences  of  atmospheric  pressure  will  be  illus- 
trated. 

Such  machines  only  as  raise  water  without  the  assistance  of 
the  atmosphere,  come  properly  under  the  present  article. 

491.  ARCHIMEDES'  SCREW. — Among  these,  one  of  the  most 
curious,  as  well  as  ancient  machines,  is  the  screw  of  Archimedes, 
and  which  was  invented  by  that  celebrated  philosopher,  two 
hundred  years  before  the  Christian  era,  and  then  employed  for 
raising  water,  and  draining  land  in  Egypt. 

492.  It  consists  of  a  tube,  made  of  lead,  or  strong  leather, 
coiled  round  a  cylinder  of  wood  or  iron,  as  represented  by  Fig. 
110.     It  has  a  support  at  each  end,  turning  on  gudgeons,  the 
upper  end  being  sometimes  furnished  with  cog-wheels  to  give  a 
more  easy  and  rapid  motion.     Both  ends  are  open,  the  lower 
one  being  placed  so  far  under  the  water  as  not  to  allow  the 
orifice  to  come  above  the  surface  in  turning  ^  the  other  dis- 
charges the  water  in  an  uninterrupted  stream. 

493.  The  angle  at  which  these  machines  work  depends  on 
the  manner  of  winding  the  tube   on  the   cylinder ;  that   is, 

Explain  Fig.  109.  490.  What  is  said  of  the  common  pump  ?  491.  Who  is  said  to 
have  been  the  inventor  of  Archimedes'  screw?  When  was  this  screw  invented? 
492.  Explain  this  machine,  as  represented  in  Fig.  110.  and  show  how  the  water  is 
elevated  by  turning  it.  493.  What  must  be  the  inclination  of  this  machine  1 


RAISING    TTATER.  129 

FIG.  110. 


whether  the  folds  touch  each  other,  or  are  at  a  distance  apart, 
for  it  is  obvious  that  if  the  tube  passes  only  a  few  times  around 
the  support,  this  must  be  in  nearly  a  horizontal  position  to  act ; 
but  if  the  folds  nearly  touch,  as  in  the  figure,  it  may  be  placed 
at  an  angle  of  about  50°  with  the  horizon.  It  will  be  apparent 
that  the  direction  of  each  fold  must  be  toward  the  horizon,  as 
the  screw  turns,  otherwise  the  water  would  not  run.  This  is 
shown  by  the  figure.  This  machine,  as  above  stated,  is  a  very 
ancient  invention,  but  has  been  re-invented  in  modern  times,  and 
employed  in  most  parts  of  Europe. 

It  has  been  constructed  in  various  ways  besides  that  here 
represented.  One  was,  to  cut  a  spiral  groove  in  a  large  log  of 
wood,  and  cover  this  with  metal,  leather,  or  boards,  so  as  to 
make  it  hold  the  water.  The  screw  being  thus  sunk  into  the 
wood,  instead  of  being  on  the  outside,  as  commonly  represented. 

494.  When  it  was  necessary  to  raise  the  water  to  a  great 
height,  a  series,  one  obliquely  above  the  other,  were  employed, 
platforms  being  constructed,  with  vessels  to  contain  the  water, 
the  lower  end  of  the  second  screw  taking  that  which  was  eleva- 
ted by  the  first ;  the  third  receiving  that  carried  up  by  the 
second,  and  so  on.     At  present  we  believe  this  engine  is  no 
where  used  except  as  a  curiosity,  there  being  better  means  of 
raising  water. 

495.  This  principle  is  readily  illustrated  by  winding  a  piece 
of  lead  tube  round  a  walking-stick,  and  then  turning  the  whole 
with  one  end  in  a  dish  of  water,  as  shown  in  the  figure. 

494.  How  was  water  raised  to  great  heights  by  this  machine!    495.  How  may  the 
principle  of  Archimedes'  screw  be  readily  illustrated  ? 

6* 


180  RAISING    WATER. 

Theory  of  Archimedes1  Screw. — By  the  following  cut  and 
•explanation,. the  manner  in  which  this  machine  acts  will  be  un- 
derstood. 

496.  Suppose  the  FIG.  in. 
extremity  1,  Fig.  Ill, 

to  be  presented  up- 
ward, as. in  the  figure, 
the  screw  itself  being 
inclined  as  represent- 
ed. Then,  from  its 
peculiar  form  and  po- 
sition, it  is  evident, 
that  commencing  at 
1,  the  screw  will  de- 
scend until  we  arrive 

at  a  certain  point,  2 ;  in  proceeding  from  2  to  3,  it  will  ascend. 
Thus,  2  is  a  point  so  situated  that  the  parts  of  the  screw  on 
both  sides  of  it  ascend,  and  therefore  if  any  body,  as  a  ball, 
were  placed  in  the  tube  at  2,  it  could  not  move  in  either  direc- 
tion without  ascending.  Again,  the  point  3  is  so  situated,  that 
the  tube  on  each  side  of  it  descends ;  and  as  we  proceed  we 
find  another  point,  4,  which,  like  2,  is  so  placed,  that  the  tube 
on  both  sides  of  it  ascends,  and,  therefore,  a  body  placed  at  4, 
could  not  move  without  ascending.  In  like  manner,  there  is  a 
series  of  other  points  along  the  tube,  from  which  it  either  de- 
scends or  ascends,  as  is  obvious  by  inspection. 

Now  let  us  suppose  a  ball,  less  in  size  than  the  bore  of  the 
tube,  so  as  to  move  freely  in  it,  to  be  dropped  in  at  1.  As  the 
tube  descends  from  1  to  2,  the  ball  of  course  will  descend  down 
to  2,  where  it  will  remain  at  rest. 

Next,  suppose  the  ball  to  be  fastened  to  the  tube  at  2,  and 
suppose  the  screw  to  be  turned  nearly  half  round,  so  that  the 
end  1  shall  be  turned  downward,  and  the  point  2  brought  to 
the  highest  point  of  the  curve  1,  2,  3. 

497.  The  last  movement  of  the  spiral,  it  is  evident,  would  so 
change  the  positions  of  the  ascending  and  descending  parts,  as 
to  -continue  the  motion   upward,  but  it  must  be  remembered 
that  the  water  differs  from  the  ball  used  for  illustration,  in  hav- 
ing a  constant  pressure  downward,  and  consequently  upward, 
and  that  the  ascent  of  the  water  depends  on  this  property  of 
the  action  of  fluids. 


4%.  Explain  the  manner  in  which  a  ball  would  ascend,  Fig.  Ill,  by  turning  the 
screw.    497.  On  what  property  of  fluids  does  the  ascent  of  the  water  depend  ? 


RAISING    WATER. 


131 


498.  BARKER'S  MILL. — For  the  different  modes  of  applying 
water  as  a  power  for  driving  mills,  and  other  useful  purposes, 
we  must  refer  the  reader  to  works  on  practical  mechanics. 
There  is,  however,  one  method  of  turning  machinery  by  water, 
invented  by  Dr.  Barker,  which  is  strictly  a  philosophical,  and, 
at  the  same  time,  a  most  curious  invention,  and  therefore  is 
properly  introduced  here. 

This  machine  is  called  Barker's  FIG.  112. 

centrifugal  mill,  and  such  parts  •  D 

of  it  as  are  necessary  to  understand 
the  principle  on  which  it  acts  are 
represented  by  Fig.  112. 

The  upright  cylinder  A,  is  a 
tube  which  has  a  funnel-shaped 
mouth  for  the  admission  of  the 
stream  of  water  from  the  pipe  B. 
This  tube  is  six  or  eight  inches  in 
diameter,  and  may  be  from  ten  to 
twenty  feet  long.  The  arms,  N 
and  O,  are  also  tubes  communica- 
ting freely  with  the  upright  one, 
from  the  opposite  sides  of  which 
they  proceed.  The  shaft  D  is 
firmly  fastened  to  the  inside  of 
the  tube,  openings  at  the  same 
time  being  left  for  the  water  to 

pass  to  the  arms  O  and  N.     The  Centrifugal  Mm. 

lower  part  of  the  tube   is  solid, 

and  turns  on  a  point  resting  on  a  block  of  stone  or  iron,  C. 
The  arms  are  closed  at  their  ends,  near  which  are  the  orifices 
on  the  sides  opposite  to  each  other,  so  that  the  water  spouting 
from  them  will  fly  in  opposite  directions.  The  stream  from  the 
pipe  B,  is  regulated  by  a  stop-cock,  so  as  to  keep  the  tube  A. 
constantly  full  without  overflowing. 

499.  To  set  this  engine  in  motion,  nothing  is  required  but 
the  force  of  the  water,  which  being  let  in  by  the  pipe,  descends, 
and  spouting  from  the  opposite  orifices,  the  motion  immediately 
begins,  and  if  the  main  tube  is  of  sufficient  length,  and  kept  full 
of  water,  it  will  in  a  few  minutes  acquire  a  whirling  velocity 
which  will  astonish  any  one  who  has  not  before  seen  this  curious 
machine. 


498.  Describe  Barker's  centrifugal  mill,  Fig.  112.    499»  How  is  this  mill   set  In 
motion  7 


132  CHAIN    PUMP. 

500.  With  respect  to  the  theory  of  its  motion,  Euler,  Greg- 
ory, Brande  and  others,  have  written ;  and  it  was  formerly  sup- 
posed to  depend  in  part  on  the  resistance  of  the  atmosphere, 
but  on  trial  it  is  said  to  revolve  most  rapidly  in  a  vacuum.  It 
is  therefore  difficult  to  explain  very  clearly  on  what  its  motion 
does  depend.  Dr.  Gregory  says,  "  In  this  machine  the  water 
does  not  act  by  its  weight,  or  momentum,  but  by  its  centrifugal 
force,  and  the  reaction  that  is  produced  by  the  flowing  of  the 
water  on  the  point  immediately  behind  the  orifice  of  discharge." 
Dr.  Brande  says,  "  The  resistance,  or  reaction  generated  by  the 
water  issuing  from  the  holes,  is  such  as  to  throw  the  vertical 
pipe  with  its  arms  and  axis  into  rapid  rotatory  motion." 

A  model  of  the  running  part  of  this  mill  may  be  made  by 
any  tinner,  for  a  few  shillings,  and  may  be  kept  in  constant  mo- 
tion, as  a  curiosity,  by  the  waste  water  from  the  water  ram  de- 
scribed a  few  pages  hence.  The  shaft  may  be  from  two  to  four 
feet. in  length,  and  an  inch  or  two  in  diameter,  the  arms  being 
one-half  or  one-third  this  size.  The  orifices  in  the  arms  must 
be  small,  otherwise  too  much  water  will  be  required,  the  quan- 
tity discharged  being  much  greater  than  might  be  supposed. 

After  a  few  revolutions,  the  machine  will  receive  an  addi- 
tional impulse  by  the  centrifugal  force  generated  in  the  arms, 
and  in  consequence  of  this,  a  much  more  violent  and  rapid  dis- 
charge of  the  water  takes  place,  than  would  occur  by  the  pressure 
of  that  in  the  upright  tube  alone.  The  centrifugal  force,  and  the 
force  of  the  discharge  thus  acting  at  the  same  time,  and  each  in- 
creasing the  force  of  the  other,  this  machine  revolves  with  great  ve- 
locity and  proportionate  power.  The  friction  which  it  has  to 
overcome,  when  compared  with  that  of  other  machines,  is  very 
slight,  being  chiefly  at  the  point  C,  where  the  weight  of  the 
upright  tube  and  its  contents  is  sustained. 

By  fixing  a  cog-wheel  to  the  shaft  at  D,  motion  may  be  given 
to  any  kind  of  machinery  required. 

Where  the  quantity  of  water  is  small,  but  its  height  consid- 
erable, this  machine  may  be  employed  to  great  advantage,  it 
being  under  such  circumstances  one  of  the  most  powerful  engines 
ever  invented. 


CHAIN    PUMP. 


501.  The  principle  of  this  machine  is  ancient,  but  instead  of 
flat  boards,  as  in  Fig.  113,  pots,  or  deep  buckets,  were  em- 

500.  What  is  the  theory  of  Barker's  mill  ?    501.  What  is  said  of  the  antiquity  and 
use  of  the  chain-pump  1    Describe  the  construction  and  action  of  this  machine. 


WATER   WHEELS. 


133 


ployed.  Such  engines  are  numerous  along  the  banks  of  tho 
Xile,  and  in  Nubia  and  Hindostan,  at  the  present  day. 

The  construction,  as  well  as  the  action  FIG.  113. 

of  the  chain-pump,  will  be  understood 
by  the  figure.  It  consists  of  a  number 
of  square  pieces  of  board,  or  of  thin 
iron,  connected  together  through  their 
centers  by  iron  rods,  so  that  they  can 
have  no  lateral  motion.  These  rods  are 
fastened  to  each  other  by  hooks  and 
eyes,  thus  forming  a  chain  with  long 
links.  The  ascending  side  of  this  chain 
passes  through  a  square  box,  to  which 
these  pieces  or  buckets  are  closely  fitted, 
but  not  so  as  to  create  much  friction. 
The  lower  wheel,  as  well  as  the  lower 
end  of  the  box,  must  be  placed  below 
the  surface  of  the  water  to  be  raised. 

The  action  of  this  machine  is  described 
in  few  words.  To  the  upper  wheel  is 
attached  a  crank ;  or  if  large  quantities 
of  water  are  to  be  raised,  as  on  board  of 
ships,  mill  work  is  added,  to  multiply 
the  motion  of  the  wheel,  in  order  to 
give  the  buckets  a  more  rapid  ascent 

through  the  box.  As  the  end  of  the  box  is  constantly  under 
the  water,  every  board  necessarily  carries  up  a  portion  in  its 
ascent,  and  although  a  single  bucket  would  elevate  but  a  small 
quantity  up  to  the  end  of  the  box,  yet  as  they  follow  each  other 
in  rapid  succession,  a  constant  stream  is  produced,  and  thus, 
when  the  trunk  is  a  foot  in  diameter,  and  the  power  is  sufficient, 
it  will  be  obvious  that  a  large  quantity  of  water  may,  in  a  short 
time,  be  elevated  by  this  means. 

602.  Although  this  machine  is  called  a  pump,  it  will  be  ob- 
served that  the  atmosphere  is  not  concerned  in  its  action. 

WATER   WHEELS. 

503.  Water  wheels  generally  consist  of  a  drum,  or  hollow 
cylinder,  revolving  on  an  axis,  while  the  diameter  or  exterior 
sui-tace  is  covered  with  flat-boards,  vanes,  or  cavities  called 

r/T2.  Does  the  chain-pump  act  by  the  pressure  of  the  atmosphere  or  not  1  503.  Of 
what  do  all  water  wheels  consist  ?  How  many  kinds  of  water  wheels  are  there,  and 
what  are  their  names  7 


134 


WATER    WHEELS. 


buckets,  upon  which  the  water  acts ;  first,  to  give  motion  to  the 
wheel,  and  then  to  machinery.  Such  wheels  are  of  three  kinds, 
namely :  the  overshot,  undershot,  and  breast  wheels. 

504.  Overshot  Wheel. — This  wheel  of  all  others,  gives  the 
greatest  power  with  the  least  quantity  of  water,  and  is,  there- 
fore, generally  used  when  circumstances  will  permit,  or  where 
there  is  a  considerable  fall,  with  a  limited  quantity  of  water. 

505.  The  overshot  wheel, 
Fig.   114,  requires  a  fall 
equal  at  least  to  its  own 
diameter,  arid  it  is  custom- 
ary  to   give   it  a  greater 
length  than  other  wheels, 
that  the  cells  or  buckets 
may  contain  a  large  quan- 
tity of  water,  for  it  is  chiefly 
by  the  weight,  and  not  the 
momentum  of  the  fluid  that 
this  wheel  is  turned. 

506.  In  its  construction, 
the  drum,  or  circumference 
is  made  water-tight,  and  to 

this     are     fixed      narrow  Overshot  Wheel. 

troughs  or  buckets,  formed 

of  iron,  or  boards,  running  the  whole  length  of  the  drum.  The 
water  is  conducted  by  a  trough  nearly  level,  and  sometimes  in 
width  equal  to  the  length  of  the  wheel.  It  falls  into  the  buckets 
on  the  top  of  the  wheel,  and  hence  the  name  overshot. 

507.  The  buckets  are  so  constructed  as  to  retain  the  water 
until  the  wheel  has  made  about  one-third  of  a  revolution  from 
the  place  of  admission,  when  it  escapes  as  from  an  inverted  ves- 
sel, and  the  wheel  ascends  with  empty  buckets,  while  on  the 
opposite  side  they  are  filled  with  water,  and  thus  the  revolution 
is  perpetuated.     This  whole  machine  and  its  action  are  so  plain 
and  obvious  as  to  require  no  particular  reference. 

508.  From  the  experiments  of  Mr.  Smeaton,  it  appears,  that 
the  fall  and  quantity  of  water,  and  the  diameter  of  the  wheel 
being  the  same,  the  overshot,  will  produce  about  double  the 
effect  of  the  undershot  wheel. 

509.  Undershot  Wheel. — This  is  so  called  because  the  water 


504.  What  is  the  chief  advantage  of  the  overshot  wheel?  505.  Is  this  wheel  turned 
by  the  weight  or  momentum  of  the  water?  506.  Describe  its  construction.  507. 
What  is  said  of  the  construction  of  the  buckets?  508.  Circumstances  being  equal, 
how  much  gre&ter  power  has  the  overshot  than  the  undershot  wheel? 


WATER    WHEELS. 


135 


Undershot  Wheel. 


passes  under  instead  of  over 
the  circumference,  as  in  that 
above  described.  Hence  it 
is  moved  by  the  momentum, 
not  the  weight  of  the  water. 

510.  Its  construction,  as 
shown  by  Fig.  115,  is  dif- 
ferent  from    the   overshot, 
since  instead  of  tight  buckets 
to  retain  the  water,  it  has 
flat-boards,    standing    like 
rays  around  the  circumfer- 
ence. 

511.  Thus    constructed, 

this  wheel  moves  equally  well  whether  the  water  acts  on  one  or 
the  other  side  cf  the  boards,  and  hence  is  employed  for  tide- 
wheels,  which  turn  in  one  direction  when  the  tide  is  going  out, 
and  in  the  other  when  it  is  coming  in. 

This  wheel  requires  a  rapid  flow,  and  a  large  quantity  of  wa- 
ter, to  give  it  an  efficient  motion. 

512.  Breast    Wheel — 
This  wheel,  in  its  construc- 
tion, or  rather  in  the  ap- 
plication  of    the    moving 
power,  is  between  the  two 
wheels   already  described. 
In  this  the  water,  instead 
of  passing  over,  or  entirely 
under  the  wheel  is  deliv- 
ered in  the  direction  of  its 
center,  Fig.  116.     This  is 
one  of  the  most  common 
wheels,    and  is   employed 
where  there  is  not  a  suffi- 
cient fall  for  the  construc- 
tion of  the  overshot  kind. 

513.  The  breast  wheel  is  moved  partly  by  the  weight,  and 
partly  by  the  momentum  of  the  water.     But  notwithstanding 
this  double  force,  this  wheel  is  greatly  inferior  to  the  overshot, 

509.  Where  does  the  water  pass  in  the  undershot  wheel?  What  kind  of  force 
moves  this  wheel  ?  510.  How  does  its  construction  differ  from  the  overshot  wheel  ? 
511.  What  is  a  tide-wheel?  512.  How  does  the  breast  wheel  differ  from  the  overshot 
and  undershot  wheels  \  Where  does  the  water  strike  this  wheel?  513.  By  what 
power  is  the  breast  wheel  moved  ?  Why  is  this  wheel  inferior  to  the  overshot  1 


FIG.  116. 


Breast  Wheel. 


136 


WATER    WHEELS. 


in  effect,  not  only  because  the  lever  power  is  diminished  by  the 
smaller  diameter,  but  also  on  account  of  the  great  waste  of  wa- 
ter which  always  attends  the  best  constructed  wheels  of  this 
kind. 

514.  General  Remarks. — In  order  to  allow  any  of  the  above 
wheels  to  act  with  freedom,  and  to  their  fullest  power,  it  is  ab- 
solutely necessary  that  the  water  which  is  discharged,  at  the 
bottom  of  the  wheel  should  have  a  wide  and  uninterrupted 
passage  to  run  away,  for  whenever  this  is  not  the  case  it  ac- 
cumulates and  forms  a  resistance  to  the  action  of  the  buckets  or 
flat- boards,  and  thus  subtracts  just  so  much  from  the  velocity 
and  power  of  the  machine. 

515.  HYDRAULIC,  OR  WATER  RAM. — This  beautiful  engine 
was  invented  by  Montgolfier,  a  Frenchman,  (and  the  same  who 
first  ascended  in  a  balloon,)  in  about  1796. 

FIG.  117. 


Hydraulic  or  Water  Ram, 

The  .form  and  construction  of  this  useful  machine,  which  is 
very  simple  in  all  its-  parts,  will  be  understood  by  Fig.  117. 
Suppose  the  pipe  A,  comes  from  a  spring,  elevated  a  few  feet 
above  the  horizontal  line  B,  and  that  it  conveys  a  constant 
stream  of  water.  At  the  termination  of  this  pipe,  there  is  a 
valve,  called  a  spindle  valve,  capable  of  closing  its  orifice  when 
drawn  upward ;  on  the  spindle  t,  are  several  small  weights,  by 
which  the  valve  is  made  to  drop  down  and  remain  open  when 
the  water  is  still ;  the  weight  of  the  whole  being  so  nicely  ad- 
justed, that  the  movement  of  the  running  water  will  elevate  it 


514.  What  cautions  are  necessary  in  order  to  permit  any  of  the  wheels  described  to 
produce  their  full  effects?  515.  Who  invented  the  hydraulic,  or  water  ram 7  Ex- 
plain its  construction  by  Fig.  117. 


WATER   WHEELS.  18Y 

to  its  place,  ana  thus  stop  the  discharge.  The  weight  of  this 
valve,  a  nice  point  in  the  construction  of  the  machine,  must  be 
just  sufficient  to  make  it  rise  by  the  force  of  the  stream,  and 
sink  again  when  the  water  ceases  to  flow,  thus  rising  and  falling, 
and  in  effect  causing  the  Laid  to  stop  for  an  instant,  and  then 
renew  its  motion. 

516.  Now  water  in  motion  acquires  a  momentum  in  propor- 
tion to  the  length  of  the  column,  and  the  height  of  the  source, 
and  when  in  action  exerts  a  force  equal  to  that  of  a  solid  body 
of  the  same  length  and  gravity,  pressing  downward  from  the 
same  elevation.     The  inelasticity  of  the  fluid  gives  it  the  prop- 
erty of  acquiring  motion  through  the  whole  length  of  a  tube 
elevated  at  one  extremity,  whenever  only  a  small  portion  is 
allowed  to  escape  by  its  own  pressure.     Hence,  when  the  valve 
opens  by  dropping  down,  all  the  water  in  the  pipe,  however 
long  it  may  be,  instantly  moves  forward  to  supply  the  place  of 
that  which  has  thus  escaped ;  and  if  the  pipe  is  long  and  the 
fountain  high,  ordinary  metallic  conductors  are  burst  asunder 
by  the  shock  whenever  the  stream  is  interrupted.     It  is  on  these 
principles  of  the  force  of  water,  that  the  Hydraulic  Ram  is 
founded ;  for  when  the  stream  is  stopped  by  the  rising  of  the 
valve,  as  already  explained,  an  outlet  is  provided  by  another 
valve,  u,  opening  upward  into  an  air  vessel,  having  a  discharg- 
ing .pipe,  x,  and  consequently  when  the  spindle  valve,  £,  is  closed, 
this  valve  instantly  opens,  and  the  water  is  thrown  with  great 
force  into  the  air  vessel,  and  through  the  discharging  pipe  to 
the  place  where  it  is  wanted.     The  stream  being  thus  inter- 
rupted, and  the  water  becoming  still  under  the  lower  valve,  this 
instantly  opens  by  falling  down,  thus  allowing  the  fluid  to  dis- 
charge itself  at.r,  when  the  motion  again  raises  the  valve,  and 
it  is  stopped,  the  valve  u  being  raised  for  its  escape  as  before ; 
and  thus  this  curious  machine,  if  well  constructed,  will  act  with 
no  other  power  or  help,  but  a  little  stream  of  water,  for  weeks 
or  months. 

517.  This  engine  affords  the  most  efficient,  cheap,  and  con- 
venient means  of  raising  water,  for  ornamental  or  farming  pur- 
poses, ever  invented.     A  spring  on  a  hill  near  the  house,  or  a 
running  brook  with  an  elevation  of  a  few  feet,  is  all  the  power 
required  to  supply  an  abundance  of  water  for  any  private,  or 
even  public  establishment.     Mr.  Millington,  who  erected  many 


516.  On  what  does  the  momentum  of  water  in  a  tube  depend  1  What  is  said  of  the 
motion  of  the  water  in  the  tube  7  517.  What  is  said  of  the  economy  and  convenience 
of  the  water  ram  7  To  what  heights  will  it  throw  water  in  proportion  to  the  falll 


140  PNEUMATICS. 

52  Y.  Expansion  of  the  Air. — On  the  contrary,  when  the 
usual  pressure  of  the  atmosphere  is  removed  from  a  portion  of 
air,  it  expands  and  occupies  a  space  larger  than  before ;  and  it 
is  found  by  experiment,  that  this  expansion  is  in  a  ratio,  as  the 
removal  of  the  pressure  is  more  or  less  cohiplete.  Air  also  ex- 
pands or  increases  in  bulk,  when  heated. 

528.  If  the  stop-cock,  C,  Fig.  118,  Be  opened,  the  piston,  A, 
may  be  pushed  down  with  ease,  because  the  air  contained  in 
the  barrel  will  be  forced  out  at  the  aperture.     Suppose  the  pis- 
ton to  be  pushed  down  to  within  an  inch  of  the  bottom,  and 
then  the  stop-cock  closed,  so  that  no  air  can  enter  below  it. 
Now,  on  drawing  the  piston  up  to  the  top  of  the  barrel,  the 
inch  of  air  will  expand  and  fill  the  whole  space,  and  were  this 
space  a  thousand  times  as  large,  it  would  still  be  filled  with  the 
expanded  air,  because  the  piston  removes  the  pressure  of  the 
external  atmosphere  from  that  within  the  barrel. 

529.  It  follows,  therefore,  that  the  space  which  a  given  por- 
tion of  air  occupies,  depends  entirely  on  circumstances.     If  it  is 
under  pressure,  its  bulk  will  be  diminished  in  exact  proportion ; 
and  as  the  pressure  is  removed,  it  will  expand  in  proportion,  so 
as  to  occupy  a  thousand,  or  even  a  million  times  as  much  space 
as  before. 

530.  Weight  of  Air. — Another  property  which  air  possesses 
is  weight,  or  gravity.     This  property,  it  is  obvious,  must  be 
slight,  when  compared  with  the  weight  of  other  bodies.     But 
that  air  has  a  certain  degree  of  gravity  in  common  with  other 
ponderous  substances,  is  proved  by  direct  experiment.     Thus  if 
the  air  be  pumped  out  of  a  close  vessel,  and  then  the  vessel  be 
exactly  weighed,  it  will  be  found  to  weigh  more  when  the  air  is 
again  admitted. 

531.  Pressure  of  the  Atmosphere. — It  is,  however,  the  weight 
of  the  atmosphere  which  presses  on  every  part  of  the  earth's 
surface,  and  in  which  we  live  and  move,  as  in  an  ocean,  that 
here  particularly  claims  our  attention. 

The  pressure  of  the  atmosphere  may  be  easily  shown  by  the 
tuba  and  piston,  Fig.  119. 

Suppose  there  is  an  orifice  to  be  opened  or  closed  by  the 
valve  B,  as  the  piston  A  is  moved  up  or  down  in  its  barrel. 
The  valve  being  fastened  by  a  hinge  on  the  upper  side,  on 

526.  In  what  proportion  ti  the  force  employed  is  the  bulk  of  air  lessened  1  527.  In 
what  proportion  will  a  quantity  of  air  increase  in  bulk  as  the  pressure  is  removed 
from  it?  528.  How  is  this  illustrated  by  Fig.  118!  529.  On  what  circumstances, 
therefore,  will  the  bulk  of  a  given  portion  of  air  depend  ?  530.  How  is  it  proved  that 
air  has  weight  ?  531.  Explain  in  what  manner  the  pressure  of  the  atmosphere  is 
shown  by  Fig.  119. 


PNEUMATICS.  141 

pushing  the  piston  aown,  it  will  open  by  the  pres-  FIG-  119- 
sure  of  the  air  against  it,  and  the  air  will  make 
its  escape.  But  when  the  piston  is  at  the  bottom 
of  the  barrel,  on  attempting  to  raise  it  again,  to- 
ward the  top,  the  valve  is  closed  by  the  force  of 
the  external  air  acting  upon  it. 

532.  If,  therefore,  the  piston  be  drawn  up  in 
this  state,  it  must  be  against  the  pressure  of  the 
atmosphere,  the  whole  weight  of  which,  to  an  ex- 
tent equal  to  the  diameter  of  the  piston,  must  be 
lifted,  while  there  will  remain  a  vacuum  or  void 
space  below  it  in  the  tube. 

533.  If  the  piston  be  only  three  inches  in  diam- 
eter, it  will  require  the  full  strength  of  a  man  to  draw  it  to  the 
top  of  the  barrel,  and  when  raised,  if  suddenly  let  go,  it  will  be* 
forced  back  again  by  the  weight  of  the  air,  and  will  strike  the 
bottom  with  great  violence. 

534.  Supposing  the  surface  of  a  man  to  be  equal  to  14-J- 
square  feet,  and  allowing  the 'pressure  on  each  square  inch  to 
be  15  Ibs.,  such  a  man  would  sustain  a  pressure  on  his  whole 
surface  equal  to  nearly  14  tons. 

Now,  that  it  is  the  weight  of  the  atmosphere  which 
presses  the  piston  down,  is  proved  by  the  fact,  that  if  its  diam- 
eter be  enlarged,  a  greater  force,  in  exact  proportion,  will  be 
required  to  raise  it.  And  further,  if  when  the  piston  is  drawn 
to  the  top  of  the  tube,  a  stop-cock,  as  at  Fig.  118,  be  opened, 
and  the  air  admitted  under  it,  the  piston  will  not  be  forced 
down  in  the  least,  because  then  the  air  will  press  as  much  on 
the  under,  as  on  the  upper  side  of  the  piston. 

535.  By  accurate  experiments,  an  account  of  which  it  is  not 
necessary  here  to  detail,  it  is  found  that  the  weight  of  the  at- 
mosphere on  every  square  inch  of  the  surface  of  the  earth  is 
equal  to  fifteen  pounds.     If,  then,  a  piston  working  air-tight  in 
a  barrel,  be  drawn  up  from  its  bottom,  the  force  employed,  be- 
sides the  friction,  will  be  just  equal  to  that  required  to  lift  the 
same  piston,  under  ordinary  circumstances,  with  a  weight  laid 
on  it  equal  to  fifteen  pounds  for  every  square  inch  of  surface. 

536.  The  number  of  square  inches  in  the  surface  of  a  piston 

532.  The  force  pressing  on  the  piston,  when  drawn  up.ward.  is  sometimes  called 
suction.  533.  Huw  is  it  proved  that  it  is  the  weight  of  the  atmosphere,  instead  of 
suction,  which  makes  the  piston  rise  with  difficulty  ?  534.  What  is  the  pressure  of 
the  atmosphere  on  the  surface  of  a  man  ?  535.  What  is  the  pressure  of  the  atmos- 
phere on  every  square  inch  of  surface  on  the  earth  ?  536.  What  is  the  number  of 
square  inches  in  a  circle  of  one  foot  in  diameter?  What  is  the  weight  of  the  atmos- 
phere on  the  surface  of  afoot  in  diameter? 


142  AIR   PUMP. 

of  a  foot  in  diameter,  is  113.  This  being  multiplied  by  the 
weight  of  the  air  on  each  inch,  which,  being  1 5  pounds,  is  equal 
to  1695  pounds.  Thus  the  air  constantly  presses  on  every  sur- 
face, which  is  equal  to  the  dimensions  of  a  circle  one  foot  in 
diameter,  with  a  weight  of  1695  pounds. 


537.  The  air  pump  is  an  engine  by  ivhich  the  air  can  be 
pumped  out  of  a  vessel,  or  withdrawn  from  it.  The  vessel  so 
exhausted,  is  called  a  receiver,  and  the  space  thus  left  in  the 
vessel,  after  withdrawing  the  air,  is  called  a  vacuum. 

Tjie  principles  on  which  the  air  pump  is  constructed  are 
readily  understood,  and  are  the  same  in  all  instruments  of  this 
^ind,  though  the  form  of  the  instrument  itself  is  often  consider- 
ably modified. 

539.  The  general  principles  of  its  FIG.  120. 

construction  will  be  comprehended 
by  an  explanation  of  Fig.  120.  Jn 
this  figure  let  R  be  a  glass  vessel,  or 
receiver,  closed  at  the  top,  and  open 
at  the  bottom,  standing  on  a  per- 
fectly smooth  surface,  which  is  called 
the  plate  of  the  air  pump.  Through 
the  plate  is  an  aperture,  which  com- 
municates with  the  inside  of  the  re- 
ceiver, and  the  barrel  of  the  pump. 
The  piston-rod  works  air-tight  Air  Pump. 

through  the  barrel.     At  the  extrem- 
ity of  the  barrel,  there  is  a  valve  which  opens  upward,  and  is 
closed  as  the  piston  rises. 

539.  Now  suppose  the  piston  to  be  drawn  up,  it  will  then 
leave  a  free  communication  between  the  receiver  R,  through  the 
orifice  to  the  pump-barrel  in  which  the  piston  works.  Then  if 
the  piston  be  forced  down,  it  will  compress  the  air  in  the  barrel 
between  V  and  V,  and,  in  consequence,  the  valve  E  will  be 
opened,  and  the  air  so  condensed  will  be  forced  out.  On  draw- 
ing the  piston  up  again,  the  valve  will  be  closed,  and  the  ex- 
ternal air  not  being  permitted  to  enter,  a  partial  vacuum  will 
be  formed  in  the  barrel,  from  V  to  V.  When  the  piston  rises 
again,  the  air  contained  in  the  glass  vessel,  together  with  that 

537.  What  is  an  air  pump  1  538.  Give  the  names  of  the  different  parts  of  the  air 
pump  by  Fig.  120.  539.  Show  the  manner  in  which  the  air  pump  works  to  produce 
a  vacuum. 


AIR   PUMP.  148 

in  the  passage  between  the  vessel  and  the  pump-barrel,  will 
rush  in  to  fill  the  vacuum.  Thus,  there  will  be  less  air  in  the 
whole  space,  and  consequently  in  the  receiver,  than  at  first,  be- 
cause all  that  contained  in  £he  barrel  is  forced  out  at  every 
stroke  of  the  piston.  On  repeating  the  same  process,  that  is 
drawing  up  and  forcing  down  the  piston,  the  air  at  each  time  in 
the  receiver  will  become  less  and  less  in  quantity,  and,  in  conse- 
quence, more  and  more  rarefied.  For  it  must  be  understood, 
that  although  the  air  is  exhausted  at  every  stroke  of  the  pump, 
that  which  remains,  by  its  elasticity,  expands,  and  still  occupies 
the  whole  space.  The  quantity  forced  out  at  each  successive 
stroke  is  therefore  diminished,  until,  at  last,  it  no  longer  has 
sufficient  force  before  the  piston  to  open  the  valve,  when  the 
exhausting  power  of  the  instrument  must  cease  entirely. 

540.  Now  it  will  be  obvious,  that  as  the  exhausting  power  of 
the  air  pump  depends  on  the  expansion  of  the  air  within  it,  a 
perfect  vacuum  can  never  be  formed  by  its  means,  for  so  long 
as  exhaustion  takes  place,  there  must  be  air  to  be  forced  out, 
and  when  this  becomes  so  rare  as  not  to  force  open  the  valves, 
then  the  process  must  end. 

DOUBLE-ACTING    AIR    PUMP. 

541.  The  double  air  pump  has  two  similar  barrels  to  that 
above  described,  and  therefore  the  process  of  exhaustion  is  per- 
formed in  half  the  time. 

This  is  represented  by  Fig.  121,  where  P  P  are  the  cylinders 
of  brass,  in  which  the  pistons  work,  and  of  which  V  V  are  the 
valves.  The  piston  rods,  E  E,  are  toothed  to  correspond  with 
the  teeth  of  the  wheel  W,  which  is  worked  by  the  crank  D. 
The  exhausting  tube  T,  also  of  brass,  opens  by  the  valves  V  V, 
into  the  cylinders.  This  has  a  stop-cock,  C,  to  prevent  the  ingress 
of  air  after  the  vacuum  is  made,  in  case  the  pistons  leak.  The 
receiver,  R,  is  of  glass,  ground  to  fit,  air-tight,  to  the  plate  of 
brass  on  which  it  stands.  The  exhausting  tube  opens  at  O, 
into  the  interior  of  the  receiver.  The  barometer  tube  H,  at  its 
upper  end  opens  into  this  tube,  while  at  the  lower  end,  M,  it  is 
inserted  into  a  cup  of  mercury. 

542.  The  barometer  tube  is  designed  to  show  the  degree  of 
exhaustion,  in  the  receiver,  with  which  it  communicates,  as 
shown  in  the  figure.     As  the  exhaustion  proceeds,  the  external 

540.  Will  the  air  pump  form  a  perfect  vacuum"?  Why  not  ?  541.  Name  the  sev- 
eral parts  of  the  double-acting  air  pump  by  Fig.  121,  and  show  how  it  works.  642. 
What  is  the  use  of  the  barometer  tube,  as  applied  to  the  air  pump! 


144 


AIR    PUMP. 
FIG.  121, 


Double-Acting  Air  Pump. 

air,  pressing  on  the  mercury  in  the  cup,  elevates  that  in  the 
tube,  in  proportion  to  the  rarety  of  the  air  in  the  receiver. 

Action. — The  manner  in  which  the  double  pump  acts,  is  ex- 
actly similar  to  the  single  one,  only  that  it  has  two  barrels,  or 
cylinders,  instead  of  one.  It  is,  therefore,  unnecessary  to  repeat 
the  explanation  given  under  the  last  figure. 

543.  External  View  of  the  Air  Pump. — Having  explained 
the  principles  and  action  of  the  air  pump,  by  figures  showing 
its  interior  construction,  we  here  present  the  student  with  an 
external  view,  Fig.  122,  of  the  whole  machine. 

544.  It  is  a  small  single-barrel  pump,  those  with  more  bar- 
rels being  of  course  more  complex  in  structure,  and  less  easily 
understood.     The  barrel,  A,  is  seven  inches  high  and  two  in 
diameter ;  the  plate,  K,  is  eight  inches  in  diameter ;  the  piston 
rod,  B,  works  air-tight  by  means  of  the  packing  screw  J,  which 
is  fitted  to  the  barrel  case,  I.     The  piston  is  kept  perpendicular 
by  the  guide  E,  through  which  it  works;  the  fulcrum  prop,  H, 
is  eighteen  inches  high,  and  the  parallel  roas,  D,  connect  the 
piston  rod  and  cross-head,  C,  with  the  lever. 

The  dome  cap,  I,  contains  a  valve  opening  upward,  for  the 
escape  of  the  air  when  the  piston  rises,  This  is  the  only  valve 
in  this  pump,  except  that  in  the  piston,  which,  as  already  shown, 
opens  to  admit  the  expanded  air  from  the  receiver,  and  force  it 

544.  Explain  all  parts  of  the  air  pump  by  Fig.  122. 


145 


Single-Barrel  Air  Pump. 


out  at  the  upper  valve.  To  the  dome  cap,  above  the  valve,  is 
fitted  a  curved  tube,  leading  to  the  cistern,  F ;  its  use  is  to  re- 
ceive the  waste  oil  which  may  escape  from  that  used  to  lubricate 
the  piston.  Th  •  globular  bell -glass,  or  receiver,  L,  is  fitted  by 
grinding  to  the  brass  plate  on  which  it  stands ;  the  barometer 
gauge,  G.  contains  mercury,  and  communicates  with  the  tube 
leading  from  the  barrel  to  the  receiver ;  this  shows  by  its  scale 
what  proportion  of  air  is  exhausted  from  the  receiver ;  within 
the  receiver  there  is  seen  a  protuberance,  showing  the  end  of 
the  exhausting  tube,  and  into  which  may  be  screwed  receivers 
or  tubes  for  various  experiments. 

545.  UPWARD  ATMOSPHERIC  PRESSURE. — The  atmosphere, 
as  we  have  seen,  presses  in  every  direction.  Its  upward  pressure 
is  shown  by  the  apparatus,  Fig.  123. 

It  consists  of  a  hand  air  pump,  a,  with  a  valve  opening  up- 
ward, not  shown.  This  pump  is  attached  to  a  cylinder  of  larger 
size,  6,  in  which  is  the  piston,  c,  to  which  a  56-lb.  weight  is 
attached  by  a  cord.  This  piston  must  be  air-tight,  and  at  the 

645.  Describe  Fig.  123,  and  show  how  the  weight  is  elevated  and  sustained. 
7 


146 


AIR    PUMP. 


Upward  Atmospheric  Pressure. 


lower  part  of  the  cylinder  when  FIG.  123. 

the  experiment  begins.  Now, 
on  working  the  pump,  a  vacu- 
um is  formed  between  the  pis- 
tons in  the  cylinder  6,  and  con- 
sequently the  pressure  of  the 
air  on  the  under  part  of  c,  the 
cylinder  being  open,  forces  it 
upward,  drawing  the  weight 
with  it.  On  admitting  the  air 
into  the  large  cylinder,  from 
above,  the  weight  instantly  de- 
scends, showing  that  it  is  the 
pressure  of  the  atmosphere 
from  below  which  sustained 
the  weight. 

546.  I.  If  a  withered  apple 
be  placed  under  the  receiver, 
and  the  air  is  exhausted,  the 
apple  will  swell  and   become 
plump,  in  consequence  of  the 

expansion  of  the  air  which  it  contains  within  the  skin. 

II.  Ether,  placed  in  the  same  situation,  soon  begins  to  boil 
without  the  influence  of  heat,  because  its  particles,  not  having 
the  pressure  of  the  atmosphere  to  force  them  together,  fly  off 
with  so  much  rapidity  as  to  produce  ebullition. 

III.  If  a  bladder  partly  filled  with  air,  and  the  neck  well  se- 
cured, has  the  external  air  exhausted,  that  within  will  so  expand 
as  to  burst  the  membrane. 

IV.  If  a  flask  partly  filled  with  water,  be  placed,  with  its 
neck  in  a  jar  of  the  same  fluid,  under  the  receiver,  the  rarefied 
air  within  the  flask  will  drive  the  water  out,  but  it  will  rush  in 
again  when  the  air  is  again  let  into  the  receiver. 

V.  If  a  burning  taper  be  placed  under  it,  the  flame  soon 
ceases  for  want  of  oxygen  to  support  it.     For  the  same  reason 
no  light  is  seen  from  the  collision  of  flint  and  steel  in  a  vacuum. 

VI.  If  a  bell  be  struck  under  the  receiver,  the  sound  will 
grow  faint  as  the  air  is  exhausted,  until  it  is  no  longer  audible. 
See  Acoustics. 

547.  Magdeburgh  Hemispheres. — One  of  the  most  striking 


546.  Why  does  an  apple  placed  in  the  exhausted  receiver  grow  plump  ?  Why  does 
ether  boil  in  the  same  situation?  Why  does  flame  cease  in  a  vacuum?  Why  is  a 
bell  inaudible  in  a  vacuum  1  547.  Describe  the  Magdeburgh  hemispheres. 


AIR    PUMP. 


147 


illustrations   of  atmospheric    pressure   is  FIG.  124. 

made  by  means  of  the  before  named  in- 
strument, Fig.  124.  It  consists  of  two 
hemispheres  of  brass,  A  and  B,  fitted  to 
each  other  by  grinding,  so  that  when  put 
together  they  perfectly  exclude  the  air. 
AYlien  put  together  without  preparation, 
or  in  the  usual  manner,  they  hold  no 
stronger  than  the  parts  of  a  snuff-box; 
but  when  the  air  is  exhausted  from  within, 
it  will  take  two  strong  men,  if  the  diam- 
eter of  the  hemispheres  are  six  inches,  to 
pull  them  apart.  The  air  is  exhausted  by 
unscrewing  the  lower  handle  and  connect- 
ing that  part  with  the  exhausting  tube  of 
the  air  pump,  and  then  by  turning  the 
key  its  return  is  prevented. 

548.  The  amount  of  force  required  to 
separate  them,  will  of  course  depend  on 

their  diameter  and  may  be  calculated  by  Mas^urgh  Hemisphere* 
estimating  the  pressure  to  be  equal  to  fif- 
teen pounds  for  every  square  inch  of  surface,  this,  as  we  have 
seen,  (536,)  being  the  pressure  of  the  atmosphere. 

549.  The  same  principle  is  involved  FIG.  125. 
when  a  piece  of  wet  leather,  with  a 

string  in  the  center,  is  pressed  on  a 
smooth  stone,  and  then  pulled  by  the 
string. 

550.  EXPANSION  FOUNTAIN. — A  very 
pretty  experiment  is  made,   with  the 
air  pump,  by  means  of  the  apparatus, 
Fig.  125. 

It  consists  of  two  glass  globes,  the 
upper  one,  a,  being  open  at  the  top, 
and  furnished  with  a  stop-cock  and  jet 
tube,  reaching  nearly  to  the  bottom  of 
the  lower  globe. 

The  lower  one,  being  nearly  filled 
with  some  colored  liquid,  the  upper 
one,  with  the  jet,  is  screwed  to  it,  as 

Seen  in  the  fio-urei.  Expansion  fountain. 


548.  What  is  the  force  required  to  pull  them  apart  1  549.  Why  does  a  piece  of  wet 
leather  adhere  to  a  smooth  surface!  550.  Explain  in  what  manner  the  fluid  in  the 
globes  is  made  to  rise,  or  fall,  at  pleasure. 


146 


AIR    PU1AP. 


Upward  Atmospheric  Pressure. 


lower  part  of  the  cylinder  when  FIG.  123. 

the  experiment  begins.  Now, 
on  working  the  pump,  a  vacu- 
um is  formed  between  the  pis- 
tons in  the  cylinder  6,  and  con- 
sequently the  pressure  of  the 
air  on  the  under  part  of  c,  the 
cylinder  being  open,  forces  it 
upward,  drawing  the  weight 
with  it.  On  admitting  the  air 
into  the  large  cylinder,  from 
above,  the  weight  instantly  de- 
scends, showing  that  it  is  the 
pressure  of  the  atmosphere 
from  below  which  sustained 
the  weight. 

546.  I.  If  a  withered  apple 
be  placed  under  the  receiver, 
and  the  air  is  exhausted,  the 
apple  will  swell  and   become 
plurnp,  in  consequence  of  the 

expansion  of  the  air  which  it  contains  within  the  skin. 

II.  Ether,  placed  in  the  same  situation,  soon  begins  to  boil 
without  the  influence  of  heat,  because  its  particles,  not  having 
the  pressure  of  the  atmosphere  to  force  them  together,  fly  off 
with  so  much  rapidity  as  to  produce  ebullition. 

III.  If  a  bladder  partly  filled  with  air,  and  the  neck  well  se- 
cured, has  the  external  air  exhausted,  that  within  will  so  expand 
as  to  burst  the  membrane. 

IV.  If  a  flask  partly  filled  with  water,  be  placed,  with  its 
neck  in  a  jar  of  the  same  fluid,  under  the  receiver,  the  rarefied 
air  within  the  flask  will  drive  the  water  out,  but  it  will  rush  in 
again  when  the  air  is  again  let  into  the  receiver. 

V.  If  a  burning  taper  be  placed  under  it,  the  flame  soon 
ceases  for  want  of  oxygen  to  support  it.     For  the  same  reason 
no  light  is  seen  from  the  collision  of  flint  and  steel  in  a  vacuum. 

VI.  If  a  bell  be  struck  under  the  receiver,  the  sound  will 
grow  faint  as  the  air  is  exhausted,  until  it  is  no  longer  audible. 
See  Acoustics. 

547.  MagdelurgJi  Hemispheres. — One  of  the  most  striking 


546.  Why  does  an  apple  placed  in  the  exhausted  receiver  grow  plump  ?  Why  does 
(ether  boil  in  the  same  situation  1  Why  does  flame  cease  in  a  vacuum  1  Why  is  a 
bell  inaudible  in  a  vacuum  1  547.  Describe  the  Magdcburgb.  hemispheres. 


AIR    PUMP. 


147 


illustrations   of  atmospheric   pressure   is  FIG.  134. 

made  by  means  of  the  before  named  in- 
strument, Fig.  124.  It  consists  of  two 
hemispheres  of  brass,  A  and  B,  fitted  to 
each  other  by  grinding,  so  that  when  put 
together  they  perfectly  exclude  the  air. 
AVlien  put  together  without  preparation, 
or  in  the  usual  manner,  they  hold  no 
stronger  than  the  parts  of  a  snuff-box; 
but  when  the  air  is  exhausted  from  within, 
it  will  take  two  strong  men,  if  the  diam- 
eter of  the  hemispheres  are  six  inches,  to 
pull  them  apart.  The  air  is  exhausted  by 
unscrewing  the  lower  handle  and  connect- 
ing that  part  with  the  exhausting  tube  of 
the  air  pump,  and  then  by  turning  the 
key  its  return  is  prevented. 

548.  The  amount  of  force  required  to 
separate  them,  will  of  course  depend  on 

their  diameter  and  may  be  calculated  by  Ma§debursk  Hemisphere* 
estimating  the  pressure  to  be  equal  to  fif- 
teen pounds  for  every  square  inch  of  surface,  this,  as  we  have 
seen,  (536,)  being  the  pressure  of  the  atmosphere. 

549.  The  same  principle  is  involved  FIG.  125. 
when  a  piece  of  wet  leather,  with   a 

string  in  the  center,  is  pressed  on  a 
smooth  stone,  and  then  pulled  by  the 
string. 

550.  EXPANSION  FOUNTAIN. — A  very 
pretty  experiment  is  made,   with  the 
air  pump,  by  means  of  the  apparatus, 
Fig.  125. 

It  consists  of  two  glass  globes,  the 
upper  one,  a,  being  open  at  the  top, 
and  furnished  with  a  stop-cock  and  jet 
tube,  reaching  nearly  to  the  bottom  of 
the  lower  globe. 

The  lower  one,  being  nearly  filled 
with  some  colored  liquid,  the  upper 
one,  with  the  jet,  is  screwed  to  it,  as 

Seen  in  the  fio*Ur€U  Expansion  Fountain. 


548.  What  is  the  force  required  to  pull  them  apart  1  549.  Why  does  a  piece  of  wet 
leather  adhere  to  a  smooth  surface!  550.  Explain  in  what  manner  the  fluid  in  the 
globes  is  made  to  rise,  or  fall,  at  pleasure. 


148 


CONDENSER. 


Thus  prepared,  they  are  placed  under  the  receiver  of  the  air 
pump,  and  as  the  air  is  exhausted,  that  contained  in  the  lower- 
globe  expands,  and  forces  the  liquid  through  the  tube  into  the 
upper  globe.  On  admitting  the  air  into  the  receiver,  the  fluid 
again  returns  into  the  lower  one,  and  this  may  be  repeated  any 
number  of  times,  affording  a  very  interesting  experiment. 


THE    CONDENSER. 


FIG.   126. 


551.  The  opera tion  of  the  condenser  is  the  reverse  of  that  of  the 
air  pump,  and  is  a  much  more  simple  machine.     The  air  pump, 
as  we  have  just  seen,  will  deprive  a  vessel  of  its  ordinary  quan- 
tity of  air.     The  condenser,  on  the  contrary,  will  double  or 
treble  the  ordinary  quantity  of  air  in  a  close  vessel,  according  to 
the  force  employed. 

This  instrument,  Fig.  126,  consists  of  a  pump- 
barrel  and  piston,  A,  a  stop-cock  B,  and  the  vessel 
C,  furnished  with  a  valve  opening  downward. 
The  orifice,  D,  is  to  admit  the  air,  when  the  pis- 
ton is  drawn  up  to  the  top  of  the  barrel. 

552.  To  describe  its  action,  let  the  piston  be 
above  D,  the  orifice  being  open,  and  therefore  the 
instrument  filled  with  air,  of  the  same  density  as 
the  external  atmosphere.     Then,  on  forcing  the 
piston  down,  the  air  in  the  pump-barrel,  below 
the  orifice  D,  will  be  compressed,  and  will  rush 
through  the  stop-cock,  B,  into  the  vessel  C,  where 
it  will  be  retained,  because,  on  again  moving  the 
piston  upward,  the  elasticity  of  the  air  will  close 
the  valve  through  which  it  was  forced.     On  draw- 
ing the  piston  up  again,  another  portion  of  air 
will  rush  in  at  the  orifice  D,  and  on  forcing  it 
clown,  this  will  also  be  driven  into  the  vessel  C ; 

and  this  process  may  be  continued  as  long  as  sufficient  force  is 
applied  to  move  the  piston,  or  there  is  sufficient  strength  in  the 
vessel  to  retain  the  air.  "When  the  condensation  is  finished,  the 
stop-cock  B  may  be  turned,  to  render  the  confinement  of  the  air 
more  secure. 

553.  AIR  GUN. — The  magazines  of  air  guns  formerly  con- 
sisted of  a  copper  ball,  which  after  being  charged  with  condensed 
air,  was  screwed  to  the  barrel,  presenting  an  unseemly  and  in- 


Condenser. 


551.  How  does  the  condenser  differ  from  the  air  pump  ?  552.  Explain  Fig.  126,  and 
show  in  what  manner  the  air  is  condensed.  553.  Explain  the  principle  of  the  air 
gun. 


CONDENSER. 


149 


convenient  appendage.     That  here   described,          TO-  127- 
is  a  more  recent  and  greatly  improved  inven- 
tion. 

In  this,  the  breach  of  the  gun  is  made  of 
copper,  and  without  much  increasing  the  size, 
answers  for  the  magazine,  while  the  barrel 
serves  as  the  tube  of  the  condenser. 

554.  At  A,  Fig.  127,  the  barrel  is  screwed 
on  to  the  breach,  in  which  the  air  is  condensed, 
by  means  of  the  piston,  rod,  and  handle,  as 
shown  by  the  figure. 

The  piston  is  then  withdrawn,  the  condensed 
air  being  prevented  from  escape  by  the  valve, 
opening  outward,  as  shown  by  the  figure. 

The  ball  being  introduced,  is  fired  by  pull- 
ing back  the  trigger,  which  opens  the  valve, 
and  allows  a  portion  of  the  air  to  escape. 

The  velocity  and  force  of  the  ball  will  de- 
pend on  the  amount  of  condensation  in  the 
magazine,  and  the  smaller  the  tube  and  piston 
by  which  this  is  made,  the  greater  of  course 
will  be  the  density  of  the  confined  air,  and  the 
more  powerful  the  force  by  which  the  ball  is 
impelled.  Where  the  piston  is  no  more  than 
half  or  three  quarters  of  an  inch  in  diameter, 
it  is  said  the  ball  will  have  a  force  not  much 
short  of  that  of  a  musket-shot. 

555.  Bottle  Imp. — A  curious  philosophical 
toy,  called  the  Bottle  Imp,  shows  in  a  very 

striking  manner  the  effects  of  condensing  a  small  portion  of  air. 

Procure  a  glass  jar,  with  a  neck,  as  represented  by  Fig.  128, 
also  a  piece  of  India  rubber,  and  a  string  to  secure  it  over  the 
mouth  of  the  jar,  so  that  it  shall  be  perfectly  air-tight.  Next, 
take  a  piece  of  glass  tube,  about  three-eighths  of  an  inch  in 
diameter,  and  with  a  file  cut  off  pieces  an  inch  long,  and  into 
one  end  of  each  put  a  cork  stopper  of  such  size  as  to  make  most 
of  the  cork  swim  on  the  surface  when  the  tube  is  placed  in  the 
water.  The  tubes  must  now  be  partly  filled  with  water,  so  that 
they  will  just  balance  themselves  in  the  fluid  without  sinking, 
the  air  remaining  in  their  upper  halves. 

Having  prepared  the  tubes  with  their  corks  in  this  manner, 


Air  Gun. 


554.  With  what  force  will  the  air  gun  fhrow  a  ball  1    555.  Explain  the  manner  of 
constructing  the  bottle  imp. 


150 


BAROMETER. 


FIG.  128. 


and  placed  them  in  the  jar  nearly  filled  with  water, 
tie  on  the  rubber  cap  with  a  good  long  string,  so 
that  no  air  can  escape,  and  this  little  apparatus  is 
finished. 

Now  press  upon  the  rubber  with  the  hand,  and 
the  floating  tubes  will  immediately  begin  to  de- 
scend, and  will  strike  the  bottom  of  the  jar,  one 
.  after  the  other,  with  an  audible  stroke,  and  will 
rise  again  when  the  pressure  ceases. 

Many  a  philosophical  head,  on  seeing  this  ex- 
periment for  the  first  time,  has  been  puzzled  to 
assign  any  cause  why  these  little  objects  should  fall 
and  rise  in  this  manner,  the  hand  not  going  near 
them,  there  being  several  inches  of  air  between  the 
cap  and  the  water. 

556.  The  explanation  will  be  obvious  on  setting 
the  jar  between  the  light  and  the  eye,  and  watch-  Bottle  imp. 
ing  a  tube  when  the  pressure  is  made,  for  the  wa- 
ter will  be  seen  to  rise  in  it  at  the  moment  it  begins  to  fall,  and 
sink  again  as  it  rises.  The  pressure  of  the  hand  is  transmitted 
through  the  elastic  rubber  and  air,  to  the  water,  and  so  to  the 
air  in  the  tube,  which  being  thus  condensed,  takes  in  more  wa- 
ter than  its  buoyancy  can  sustain,  and  it  sinks — rising  again 
when  the  air  is  allowed  to  expand,  and  drive"  out  the  water. 


BAROMETER. 

55*7.  The  Barometer  is  an  instrument  which,  by  means  of  a 
column  of  mercury  in  a  glass  tube,  shows,  by  its  elevation  and 
depression,  the  different  degrees  of  atmospheric  pressure. 

558.  Suppose  A,  Fig.  129,  to  be  a  long  tube,  with  the  piston 
B  so  nicely  fitted  to  its  inside,   as  to  work  air-tight.     If  the 
lower  end  of  the  tube  be  dipped  into  water,  and  the  piston 
drawn  up  by  pulling  at  the  handle  C,  the  water  will  follow  the 
piston  so  closely,  as  to  be  in  contact  with  its  surface,  and  ap- 
parently to  be  drawn  up  by  the  piston,  as  though  the  whole 
was  one  solid  body.     If  the  tube  be  thirty-five  fejet  long,  the 
water  will  continue  to  follow  the  piston,  until  it  comes  to  the 
height  of  about  thirty-three  feet,  where  it  will  stop. 

559.  If  the  piston  be  drawn  up  still  further,  the  water  will 

556.  Explain  the  reason  why  the  floats  in  the  water  imp  are  influenced  by  the  pres- 
sure 557.  What  is  the  barometer  ?  558.  Supi  ose  the  tube,  Fitf.  129,  to  stand  with 
its  lower  end  in  the  water,  and  the  piston  A  to  be  drawn  upward  thirty  five  feet,  how 
far  will  the  water  follow  the  piston  1  559.  What  will  remain  in  the  tibe  between  the 
piston  and  the  water,  after  the  piston  rises  higher  than  thirty-three  teet  1 


BAROMETER. 


151 


FIG.  129. 


m 


not  follow  it,  but  will  remain  stationary,  the 
space  from  this  height,  between  the  piston  and 
the  water,  being  left  a  void  space  or  vacuum. 

560.  The  rising  of  the  water  in  the  above 
case,  which  only  involves  the  principle  of  the 
common  pump,  is  thought  by  some  to  be  caused 
by  suction,  the  piston  sucking  up  the  water  as  it  is 
drawn  upward.     But  according  to  the  common 
notion  attached  to  this  term,  there  .is  no  reason 
why  the  water  should  not  continue  to  rise  above 
the"  thirty-three  feet,  or  why  the  power  of  suc- 
tion should  cease  at  that  point,  rather  than  at 
any  other. 

561.  Without  entering  into  any  discussion 
on  the  absurd  notions  concerning  this  power,  it 
is  sufficient  here  to  state,  that  it  has  long  since 
been  proved,  that  the  elevation  of  the  water,  in 
the  case  'above  described,  depends  entirely  on 
the  weight  and  pressure  of  the  atmosphere  on 
that  portion  of  the  fluid  which  is  on  the  outside 
of  the  tube.     Hence,  when  the  piston  is  drawn 
up  under  circumstances  where  the  air  can  not 
act  on  the  water  around  the  tube,  or  pump- 
barrel,  no  elevation  of  the  fluid  will  follow. 

562.  If  an  atmospheric  pump,  or  even  the  suction-hose  of  a 
fire  engine,  be  inserted  into  the  side  of  a  tight  cask  filled  with 
fluid,  all  the  force  of  what  is  called  suction  may  be  exerted  by 
the  pump  or  engine  in  vain ;  for  the  liquid  will  not  rise  until  an 
aperture,  admitting  the  atmosphere,  is  made  in  some  part  of  the 
cask.     It  may  be  objected  that  wells,  though  covered  several 
feet  deep  with  earth,  still  admit  water  to  be  drawn  from  them 
by  pumps,  with  all  the  facility  of  those  which  are  open.     But  it 
must  be  remembered  that  the  ground  is  porous,  admitting  the 
atmosphere  to  an  unknown  depth  from  the  surface,  and  hence 
wells  can  not  be  covered  by  any  common  means  so  as  to  ex- 
clude sufficient  atmospheric  pressure  for  the  purpose  in  question. 
That  the  pump  will  not  raise  water  without  the  influence  of  the 
atmosphere,  will  be  seen  by  the  following  experiment. 

503.  Proof  that  the  Pump  acts  by  External  Pressure. — 
Suppose  Fig.  130  to  be  the  sections  or  halves,  of  two  tubes,  one 

560.  What  is  commonly  supposed  to  make  the  water  rise  in  such  cases'?  Is  there 
any  reason  why  the  suction  should  cease  at  thirty-three  feet  ?  561.  What  is  the  true 
cause  of  the  elevation  of  the  water,  when  the  piston,  Fig.  129,  is  drawn  up  >  562. 
Will  the  suction-hose  of  a  fire-engine  raise  water  from  a  tight  cask  7 


152 


BAROMETER. 


within  the  other,  the  outer  one  being  made  en- 
tirely close,  so  as  to  admit  no  air,  and  the  space 
between  the  two  being  also  made  air-tight  at  the 
top.  Suppose,  also,  that  the  inner  tube  being  left 
open  at  the  lower  end,  does  not  reach  the  bottom 
of  the  outer  tube,  and  thus  that  an  open  space  be 
left  between  the  two  tubes  every  where,  except  at 
their  upper  ends,  where  they  are  fastened  to- 
gether ;  and  suppose  that  there  is  a  valve  in  the 
piston,  opening  upward,  so  as  to  let  the  air  which 
it  contains  escape,  but  which  will  close  on  draw- 
ing the  piston  upward.  Now,  let  the  piston  be 
at  A,  and  in  this  state  pour  water  through  the 
stop-cock,  C,  until  the  inner  tube  is  filled  up  to 
the  piston,  and  the  space  between  the  two  tubes 
filled  up  to  the  same  point,  and  then  let  the  stop- 
cock be  closed.  If  now  the  piston  be  drawn  up 
to  the  top  of  the  tube,  the  water  will  not  follow 
it,  as  in  the  case  of  Fig.  129 ;  it  will  only  rise  a 
few  inches,  in  consequence  of  the  elasticity  of  the 
air  above  the  water,  between  the  tubes,  and  in 
the  space  above  the  water,  there  will  be  formed  a 
vacuum  between  the  water  and  the  piston,  in  the 
inner  tube. 

The  reason  why  the  result  of  this  experiment  differs  from  that 
before  described,  is,  that  the  outer  tube  prevents  the  pressure  of 
the  atmosphere  from  forcing  the  water  up  the  tube  as  th«  piston 
rises.  This  may  be  instantly  proved,  by  opening  the  stop-cock 
C,  and  permitting  the  air  to  press  upon  the  water,  when  it  will 
be  found,  that  as  the  air  rushes  in,  the  water  will  rise  and  fill 
the  vacuum,' up  to  the  piston. 

564.  For  the  same  reason,  if  a  common  pump  be  placed  in  a 
cistern  of  water,  and  the  water  is  frozen  over  on  its  surface,  so 
that  no  air  can  press  upon  the  fluid,  the  piston  of  the  pump 
might  be  worked  in  vain,  for  the  water  would  not,  as.  usual, 
obey  its  motion. 

,  565.  It  follows,  as  a  certain  conclusion  from  such  experi- 
ments, that  when  the  lower  end  of  a  tube  is  placed  in  water, 
and  the  air  from  within  removed  by  drawing  up  the  piston, 
that  it  is  the  pressure  of  the  atmosphere  on  the  water  around 

563.  How  is  it  shown  by  Fig.  130.  that  it  is  the  pressure  of  the  atmosphere  wh'ch 
causes  the  water  to  rise  in  the  pump-barrel  ?  564.  Suppose  the  ice  prevents  the  at 
mosplvre  from  pressing  on  the  water  in  a  vessel,  can  the  water  be  pumped  out  ? 
665.  What  conclusion  follows  from  the  experiments  above  described? 


BAROMETER. 


153 


the  tube,  which  forces  the  fluid  up  to  fill  the  space  thus  left  by 
the  air. 

566.  It  is  also  proved,  that  the  weight,  or  pressure  of  the  at- 
mosphere, is  equal  to  the  weight  of  a  perpendicular  column  of 
water  33  feet  high,  for  it  is  found  (Fig.  129)  that  the  pressure 
of  the  atmosphere  will  not  raise  the  water  more  than  33  feet, 
though  a  perfect  vacuum  be  formed  to  any  height  above  this 
point 

567.  Experiments   on  other  fluids,  prove  -that  this  is  the 
weight  of  the  atmosphere,  for  if  the  end  of  the  tube  be  dipped 
in  any  fluid,  and  the  air  be  removed  from  the  tube,  above  the 
fluid,  it  will  rise  to  a  greater  or  less  height  than  water,  in  pro- 
portion as  its  specific  gravity  is  less,  or  greater  than  that  fluid. 

568.  Mercury,  or  quicksilver,  has  a  specific  gravity  of  about 
13£  times  greater  than  that  of  water,  and  mercury  is  found  to 
rise  about  29  inches  in  a  tube  under  the  same  circumstances  that 
water  rises  33  feet.     Xow,  33  feet  is  396  inches,  which  being 
divided  by  29,  gives  nearly  13-^,  so  that  mercury  being   13-J- 
times  heavier  than  water,  the  water  will  rise  under  the  same 
pressure  13-£  times  higher  than  the  mercury. 

569.  Construction  of  the  Barometer. — 
The  barometer  is  constructed  on  the  princi- 
ple of  atmospheric  pressure.     This  term  is 
compounded    of  two    Greek    words,   baros, 
weight,  and  metron,  measure,  the  instrument 
being  designed  to  measure  the  weight  of  the 
atmosphere. 

Its  construction  is  simple  and  easily  un- 
derstood, being  m4My  a  tube  of  glass,  nearly 
filled  with  mercury,  with  its  lower  end  placed 
in  a  dish  of  the  same  fluid,  and  the  upper 
end  furnished  with  a  scale,  to  measure  the 
height  of  the  mercury. 

570.  Let  A,  Fig.  131,  be  such  a  tube, 
thirty-four  or  thirty-five  inches  long,  closed 
at  one  end,  and  open  at  the  other.     To  fill 
the  tube,  set  it  upright,  and  pour  the  mer- 
cury in  at  the  open  end,  and  when  it  is  en- 
tirely full,  place  the  fore-finger  forcibly  on 


FIG.  131. 


Barometer. 


566.  How  is  it  proved  that  the  pressure  of  the  atmosphere  is  equal  to  the  weight  of 
a  column  of  water  33  feet  hisrh  ?  567.  How  do  experiments  on  other  fluids  show  that 
the  pressure  of  the  atmosphere  is  equal  to  the  weight  of  a  column  of  water  33  feet 
hijih  }  563.  How  hish  does  mercury  rise  in  an  exhausted  tube?  How  does  the 
height  of  mercury  in  the  barometer  indicate  that  of  water  I 

7* 


154  BAROMETER. 

this  end,  and  then  plunge  the  tube  and  finger  under  the  surface 
of  the  mercury,  before  prepared  in  the  cup,  B.  Then  withdraw 
the  finger,  taking  care  that  in  doing  this,  the  end  of  the  tube  is 
not  raised  above  the  mercury  in  the  cup.  When  the  finger 
i*  removed,  the  mercury  will  descend  four  or  five  inches,  and 
after  several  vibrations,  up  and  down,  will  rest  at  an  elevation 
of  29  or  30  inches  above  the  surface  of  that  in  the  cup,  as  at  C. 
Having  fixed  a  scale  to  the  upper  part  of  the  tube,  to  indicate 
the  rise  and  fall  of  the  mercury,  the  barometer  would  be  fin- 
ished, if  intended  to  remain  stationary.  It  is  usual,  however,  to 
have  the  tube  inclosed  in  a  mahogany  or  brass  case,  to  prevent 
its  breaking,  and  to  have  the  cup  closed  at  the  top,  and  fastened 
to  the  tube,  so  that  it  can  be  transported  without  danger  of 
spilling  the  mercury. 

571.  Cup  of  the  Portable  Barometer. — The  cup  of  the  port- 
able barometer  also  differs  from  that  described,  for  were  the 
mercury  inclosed  on  all  sides,  in  a  cup  of  wood,  or  brass,  the  air 
would  be  prevented  from  acting  upon  it,  and  therefore  the  in- 
strument would  be  useless.     To  remedy  this  defect,  and  still 
have  the  mercury  perfectly  inclosed,  the  bottom  of  the  cup  is 
made  of  leather,  which,  being  elastic,  the  pressure  of  the  atmos- 
phere acts  upon  the  mercury  in  the  same  manner  as  though  it 
was  not  inclosed  at  all. 

572.  Below  the  leather  bottom,  there  is  a  round  plate  of 
metal,  an  ift^.in  diameter,  which  is  fixed  on  the  top  of  a  screw, 
so  that  when  the  instrument  is  to  be  transported,  by  elevating 
this  piece  of  metal,  the  mercury  is  thrown  up  to  the  top  of  the 
tube,  and  thus  kept  from  playing  backward  ^jid  forward,  when 
the  barometer  is  in  motion. 

573.  A  person  not  acquainted  with  the  principles  of  this  in- 
strument, on  seeing  the  tube  turned  bottom  upward,  will  be 
perplexed  to  understand  why  the  mercury  does  not  follow  the 
common  law  of  gravity,  and  descend  into  the  cup ;  were  the 
tube  of  glass  33  feet  high,  and  filled  with  water,  the  lower  end 
being  dipped  into  a  tumbler  of  the  same  fluid,  the  wonder  would 
be  still  greater.     But  as  philosophical  facts,  one  is  no  more 
wonderful  than  the  other,  and  both  are  readily  explained  by  th<> 
principles  above  illustrated. 


569.  What  is  the  principle  on  which  the  barometer  is  constructed  ?  570.  Describe 
the  construction  of  the  barometer,  as  represented  by  Fig.  131.  571.  How  is  the  cup 
of  the  portable  barometer  made  so  as  to  retain  the  mercury,  and  still  allow  the  air  to 
press  upon  it  ?  572.  What  is  the  use  of  the  metallic  plate  and  screw,  under  the  bot- 
tom of  the  cup  ?  573.  Explain  the  reason  why  the  mercury  does  not  fall  out  of  the 
barometer  tube  when  its  open  end  is  downward. 


BAROMETER.  155 

574.  WATER  BAROMETER. — It  has  already  been  shown,  (563,) 
that  it  is  the  pressure  of  the  atmosphere  on  the  fluid  around 
the  tube,  bv  which  the  fluid  within  it  is  forced  upward,  when 
the  pump  is  exhausted  of*  its  air.     The  pressure  of  the  air,  we 
have  also  seen,  is  equal  to  a  column  of  water  33  feet  high,  or 
of  a  column  of  mercury  29  inches  high.     Suppose,  then,  a  tube 
33  feet  high  is  filled  with  water,  the  air  would  then  be  entirely 
excluded,  and  were  one  of  its  ends  closed,  and  the  other  end 
dipped  in  water,  the  effect  would  be  the  same  as  though  both 
ends  were  closed,  for  the  water  would  not  escape,  unless  the  air 
was  permitted  to  rush  in  and  fill  up  its  place.     The  upper  end 
being  closed,  the  air  could  gain  no  access  in  that  direction,  and 

'the  open  end  being  under  water,  is  equally  secure.  The  quan- 
tity of  water  in  which  the  end  of  the  tube  is  placed,  is  not  essen- 
tial, since  the  pressure  of  a  column  of  water,  an  inch  in  diameter, 
provided  it  be  33  feet  high,  is  just  equal  to  a  column  of  air  of 
an  inch  in  diameter,  of  the  whole  height  of  the  atmosphere. 
Hence  the  wnter  on  the  outside  of  the  tube  serves  merely  to 
guard  against  the  entrance  of  the  external  air. 

575.  The  same  happens  to  the  barometer  tube,  when  filled 
with  mercurv.     The   mercury,  in  the  first  place,  fills  the  tube 
perfectly,  and  therefore  entirely  excludes  the  air,  so  that  when 
it  is  inverted  in  the  cup  or  cistern,  all  the  space  above  29  inches 
is  left  a  vacuum.     The  same  effect  precisely  would  be  produced, 
were  the  tube  exhausted  of  its  air,  and  the  open  end  placed  in 
the  cup ;  the  mercury  would  run  up  the  tube  29  inches,  and 
then  stop,  all  above  that  point  being  left  a  vacuum. 

576.  The  mercury,  therefore,  is  prevented  from  falling  out 
of  the  tohpi,  by  the  pressure  of  the  atmosphere  on  that  which 
remains  in  the  cistern  ;  for  if  this  be  removed,  the  air  will  enter, 
while  the  mercury  will  instantly  begin  to   descend.     This  is 
called  the  Cistern  barometer. 

577.  WHEEL  BAROMETER. — In  the  common  barometer,  the' 
rise  and  fell  of  the  mercury  is  indicated  by  a  scale  of  inches,  and 
tenths  of  inches,  fixed  behind  the  tube ;  but  it  has  been  found 
that  vpry  slight  variations  in  the  density  of  the  atmosphere  are 
not  readily  perceived  by  this  method.     It  being,  however,  de- 
sirable that  these  minute  changes  sh'ould  be  rendered  more 
obvious,  a  contrivance  for  increasing  the  scale,  called  the  wheel 
barometer,  was  invented. 

574.  How  high  does  the  fluid  stand  in  the  water  barometer  7  575.  What  fills  the 
space  above  29  inches,  in  the  barometer  tube?  576.  What  prevents  the  mercury 
from  falling  out  of  the  barometer  tube!  577.  In  the  common  barometer,  how  is  the 
rise  and  fall  of  the  mercury  indicated  ?  Why  was  the  wheel  barometer  invented  1 


156 


BAROMETER. 


578.  The  whole  length  of  the  tube  of  the 
wheel  barometer,  Fig.  132,  from  C  to  A,  is  34 
or  35  inches,  and  it  is  filled  with  mercury,  as 
usual.     The  mercury  rises  in  the  short  leg  to 
the  point  o,  where  there  is  a  small  piece  of 
glass  floating  on   its  surface,   to  which  there 
is  attached  a  silk  string,  passing  over  th.e  pul- 
ley p.     To  the  axis  of  the  pulley  is  fixed  an 
index,  or  hand,  and  behind  this  is  a  graduated 
circle,  as  seen  in  the  figure.     It  is  obviouS,  that 
a  very  slight  variation  in  the  height  of  the 
mercury  at  o,  will  be  indicated  by  a  considera- 
ble motion  of  the"  index,  and  thus  changes  in 
the  weight  of  the  atmosphere,  hardly  percepti- 
ble by  the  common  barometer,   will  become 
quite  apparent  by  this. 

579.  Heights  Measured  by  the  Barometer. — 
The  mercury  in  the  barometer  tube  being  sus- 
tained by  the  pressure  of  the  atmosphere,  and 
its  medium  altitude  at  the  surface  of  the  earth 

being  29  to  30  inches,  it  might  be  expected  that  if  the  instru- 
ment was  carried  to  a  height  from  the  earth's  surface,  the  mer- 
cury would  suffer  a  proportionate  fall,  because  the  pressure  must 
be  less  at  a  distance  from  the  earth,  than  at  its  surface,  and  ex- 
periment proves  'this  to  be  the  case.  When,  therefore,  this 
instrument  is  elevated  to  any  considerable  height,  the  descent 
of  the  mercury  becomes  perceptible.  Even  when  it  is  carried 
to  the  top  of  a  hill,  or  high  tower,  there  is  a  sensible  depression 
of  the  fluid,  so  that  the  barometer  is  employed  to  measure  the 
height  of  mountains  and  the  elevation  to  which  balloons  ascend 
from  the  surface  of  the  earth.  On  the  top  of  Mont  Blanc, 
which  is  about  16,000  feet  above  the  level  of  the  sea,  the  me- 
dium elevation  of  the  mercury  in  the  tube  is  only  14  inches, 
while  on  the  surface  of  the  earth,  as  above  stated,  it  is  29  to 
30  inches. 

580.  Diminution  of  Density. — The  following  table  shows  at 
what  rate  the  atmosphere  decreases  in  density,  as  indicated  by 
the  barometer.     A  part  of  these  numbers  are  from  actual  ob- 


Wheel  Barometer. 


578.  Explain  Fig.  132,  and  describe  the  construction  of  the  wheel  barometer.  5~9. 
What  is  stated  to  be  the  medium  ranjre  of  the  barometer  at  the  surface  of  the  earth  1 
Suppose  the  instrument  is  elevated  from  the  earth,  what  is  the  effect  on  the 
mercury?  How  does  the  barometer  indicate  the  height  of  mountains?  580.  Explain 
by  the  table  the  correspondence  between  the  height,  the  fall  of  the  mercury,  and  the 
temperature. 


BAROMETER. 


servations  made  from  ascents  in  balloons,  and  a  part  from  esti- 
mates. The  medium  pressure  of  the  atmosphere,  at  the  level 
of  the  sea,  is  estimated  at  30  inches,  and  is  expressed  by  0. 


HEIGHT    IN    MILES. 

PRESSURE. 

TEMPERATURE. 

Inches. 

Fahr. 

0 

30.00 

50.0 

1 

24.61 

35.0 

2 

20.07 

19.5 

3 

16.35 

3.4 

4 

13.06 

13.3 

5 

10.41 

30.6 

10 

2.81 

126.4 

15 

.45 

240.6 

Thus,  according  to  this  estimate,  at  the  height  of  fifteen  miles, 
the  mercury  falls  to  less  than  half  an  inch,  while  the  cold  is 
equal  to  240  degrees  below  the  zero  of  Fahrenheit. 

581.  Principles  of  the    Barometer  applied   to    the    Water 
Pump. — As  the  efficacy  of  the  pump  depends  on  the  pressure 
of  the  atmosphere,  the  barometer  will  always  indicate  the  height 
to  which  it  can  be  effectual  at  any  given  place.     Thus,  on  Mont 
Blanc,  where  the  barometer  stands  at  only  14  inches,  being  less 
than  one-half  its  height  on  the  sea  level,  the  water  pump  would 
only  raise  the  fluid  about  1 5  feet.     Hence,  engineers  and  others, 
who  visit  elevated  countries,  should  calculate  by  the  barometer, 
from  what  depth  they  can  raise  water  by  aerial  pressure,  before 
they  erect  works  for  this  purpose. 

At  the  city  of  Mexico  and  at  Quito,  for  instance,  the  suction 
tube  can  only  act  to  the  depth  of  22  or  20  feet,  while  on  the 
Himalay  mountains  its  rise  will  be  only  about  8  or  10  feet. 

582.  USE   AS  A  WEATHER  GLASS. — While   the   barometer 
stands  in  the  same  place,  near  the  level  of  the  sea,  the  mercury 
seldom  or  never  falls  below"  28  inches,  or  rises  above  31  inches; 
its  whole  ran^e,  while  stationary,  being  only  about  3  inches. 

583.  These  changes  in  the  weight  of  the  atmosphere,  indf- 
cate  corresponding  changes  in  the  weather,  for  it  is  found,  by 
watching  these  variations  in  the  height  of  the  mercury,  that 
when  it  falls,  cloudy  or  falling  weather  ensues,  and  that  when 

581.  How  hish  will  the  pump  raise  water  on  Mont  Blanc?  To  what  height  in 
Mexico  and  Quito  ?  582.  How  many  inches  does  a  fixed  barometer  vary  in  height  1 
5S3  When  the  mercury  falls,  what  kind  of  weather  is  indicated  ?  When  the  mer- 
cury r  s~s.  what  kind  of  weather  may  be  expected  ?  When  fog  and  smoke  descenj 
toward  the  ground,  is  it  a  sign  of  a  light  or  heavy  atmosphere  1 


158  BAROMETER. 

it  rises,  fine  clear  weather  may  be  expected.  During  the  time 
when  the  weather  is  damp  and  lowering,  and  the  smoke  of 
chimneys  descends  toward  the  ground,  the  mercury  remains  de- 
pressed, indicating  that  the  weight  of  the  atmosphere,  during 
such  weather,  is  less  than  it  is  when  the  sky  is  clear.  This  con- 
tradicts the  common  opinion,  that  the  air  is  the  heaviest  when 
it  contains  the  greatest  quantity  of  fog  and  smoke,  and  that  it 
is  the  uncommon  weight  of  the  atmosphere  which  presses  these 
vapors  toward  the  ground. 

584.  A  little  consideration  will  show,  that  in  this  case  the 
popular  belief  is  erroneous,  for  not  only  the  barometer,  but  all 
the  experiments  we  have  detailed  on  the  subject  of  specific  grav- 
ity, tend  to  show  that  the  lighter  any  fluid  is,  the  deeper  any 
substance  of  a  given  weight  will  sink  in  it.     Common  observa- 
tion ought,  therefore,  to  correct  this  error,  for  every  body  knows 
that  a  heavy  body  will  sink  in  water,  while  a  light  one  will 
swim,  and  by  the  same  kind  of  reasoning  ought  to  consider, 
that  the  particles  of  vapor  would  descend  through  a  light  atmos- 
phere, while  they  would  be  pressed  up  into  the  higher  regions 
by  a  heavier  air. 

585.  The  following  indications  of  the  barometer  with  respect 
to  the  weather,  may  be  depended  on  as  correct,  being  tested  by 
the  observations  of  the  author : — 

I.  In  calm  weather,  when  the  wind,  clouds,  or  sun,  indicate 
approaching  rain,  the  mercury  in  the  barometer  is  low. 

II.  In  serene,  fine,  settled  weather,  the  mercury  is  high,  and 
often  remains  so  for  days. 

III.  Before  great  winds,  and  during  their  continuance,  from 
whatever  quarter  they  come,  the  mercury  sinks  lowest,  and 
especially  if  they  come  from  the  south. 

IV.  During  the  coldest,  clear  days,  when  a  gentle  wind  from 
the  north  or  west  prevails,  the  mercury  stands  highest. 

V.  After  great  storms,  when  the  mercury  has  been  lowest,  it 
rises  most  rapidly. 

VI.  It  often  requires  considerable  time  for  the  mercury  tc 
gain  its  wonted  elevation  after  a  storm  ;  and  on  the  contrary, 
it  sometimes  rains  without  the  usual  corresponding  change  in 
its  altitude. 

VII.  In  general,  whether  there  are  any  appearances  of  change 
in  the  horizon  or  not,  we  may  prognosticate  rain  whenever  the 
mercury  sinks  during  fine  weather. 

584.  By  what  analogy  is  it  shown  that  the  air  is  lightest  when  filled  with  vapor  1 
685.  Mention  the  indications  of  the  barometer  concerning  the  weather. 


LIB/ 

WATER    PUMPS.  JT  ^V^ffcr  TEE 

[.  When  it  rains  with  the  mercury  high,  w$  may  be  sure 
that  it  will  soon  be  fair. 

586.  USE  AT  SEA. — The  principal  use  of  the  baroi 
board  of  ships,  where  it  is  employed  to  indicate  the  aj 
of  storms,  and  thus  to  give  an  opportunity  of  preparing  accord- 
ingly ;  and  it  is  found  that  the  mercury  suffers  a  most  remark- 
able depression  before  the  approach  of  violent  winds,  or  hurri- 
canes.    The  watchful  captain,  particularly  in  southern  latitudes, 
is  always  attentive  to  this  monitor,  and  when  he  observes  the 
mercury  to  sink  suddenly, 'takes  his  measures  without  delay  to 
meet  the  tempest.     During  a  violent  storm,  we  have  seen  the 
wheel  barometer  sink  a  hundred  degrees  in  a  few  hours. 

58.7.  Preservation  by  the  Barometer. — But  we  can  not  illus- 
trate the  use  of  this  instrument  at  sea  better  than  to  give  the 
following  extract  from  Dr.  Arnot,  who  was  himself  present  at 
the  time.  "  It  was,"  he  says,  "  in  a  southern  latitude.  The  sun 
had  just  set  with  a  placid  appearance,  closing  a  beautiful  after- 
noon, and  the  usual  mirth  of  the  evening  watch  proceeded, 
when  the  Captain's  orders  came  to  prepare  with  all  haste  for  a 
storm.  The  barometer  had  begun  to  fall  with  appalling  ra- 
pidity. As  yet,  the  oldest  sailors  had  not  perceived  even  a 
threatening  in  the  sky,  and  were  surprised  at  the  extent  and 
hurry  of  the  preparations  ;  but  the  required  measures  were  not 
completed,  when  a  more  awful  hurricane  burst  upon  them  than 
the  most  experienced  had  ever  braved.  Nothing  could  with- 
stand it ;  the  sails,  already  furled,  and  closely  bound  to  the 
yards,  were  riven  into  tatters ;  even  the  bare  yards  and  masts 
were  in  a  great  measure  disabled ;  and  at  one  time  the  whole 
rigging  had  nearly  fallen  by  the  board.  Such,  for  a  few  hours, 
was  the  mingled  roar  of  the  hurricane  above,  of  the  waves 
around,  and  the  incessant  peals  of  thunder,  that  no  human  voice 
could  be  heard,  and  amidst  the  general  consternation,  even  the 
trumpet  sounded  in  vain.  On  that  awful  night,  but  for  a  little 
tube  of  mercury  which  had  given  the  warning,  neither  the 
strength  of  the  noble  ship,  nor  the  skill  and  energies  of  her 
commander,  could  have  saved  one  man  to  tell  the  tale." 

WATER    PUMPS. 

588.  The  efficacy  of  the  common  pump  in  raising  water,  de- 
pends upon  the  force  of  atmospheric  pressure,  the  principles  of 

586.  Of  what  use  is  the  barometer  on  board  of  ships!  When  does  the  mercury 
suffer  the  most  remarkable  depression  1  5S7.  What  remarkable  instance  is  stated, 
where  a  ship  seemed  to  be  saved  by  the  use  of  the  barometer  1  583.  On  what  does 
th«  efficacy  of  the  common  pump  depend  ] 


160 


WATER    PUMPS. 


FIG.  133. 


which  have  been  fully  illustrated  under  the  articles,  Air  Pump 
and  Barometer. 

589.  An  experiment,  of  jvhich  few  are  ignorant,  and  which 
all  can  make,  shows  the  principle  of  the  pump  in  a  very  strik- 
ing manner.     If  one  end  of  a  straw  be  dipped  into  a  vessel  of 
liquid,  and  the  other  end  be  sucked,  the  liquid  will  rise  into  the 
mouth,  and  may  be  swallowed. 

The  principles  which  this  experiment  involves  are  exactly  the 
same  as  those  concerned  in  raising  water  by  the  pump.  The 
vessel  of  liquid  answers  to  the  well,  the  straw  to  the  pump  log, 
and  the  mouth  acts  as  the  piston,  by  which  the  air  is  removed. 

Water  pumps  are  of  three  kinds,  namely,  the  sucking,  or  com- 
mon pump,  the  lifting  pump,  arid  the  forcing  pump. 

590.  COMMON  METALLIC 
PUMP.— This    (Fig.     133,) 
consists  of  a  brass  or  iron  bar- 
rel, A,  containing  at  its  up- 
per part  a  hollow  piston  and 
valve,  opening  upward.  Be- 
low  this   there   is    another 
valve,  also  opening  upward. 
The  pipe  and  stop-cock  C, 
are  for  the  purpose  of  letting 
the  water  from  the  barrel  to 
the  tube,  which  descends  into 
the  well. 

The  action  of  this  pump 
depends  on  the  pressure  of 
the  atmosphere,  and  will  be 
readily  understood  by  the 
pupil  who  has  learned  what 
is  said  under  the  articles  air 
pump  and  barometer. 

591.  On  raising  the  lever, 
D,  the  piston,  A,  descends 
down  the  barrel,  the  lower 
valve,  B,  at  the  same  mo- 
ment Closing  by  the  weight  Common  Metallic  Pump. 

of  the  water,  while  the  up- 


5S9.  What  experiment  is  stated,  as  illustrating  the  principle  of  the  common  pump  1 
How  many  kinds  of  pumps  are  mentioned  1  590.  Which  kind  is  the  common  1  De- 
scribe the  common  pump.  Explain  how  the  common  pump  acts.  591.  When  the 
ever  is  raised,  what  takes  place  in  the  pump-barrell  When  it  is  depressed,  what 
,akes  place  ? 


WATER    PUMPS. 


161 


FIG.  134. 


per  one  opens  and  lets  the  water  through.  Then,  on  depress- 
ing the  lever,  the  piston  rises,  its  valve  closing,  and  elevating 
the  water  above  it.  By  this  action  a  vacuum  would  be  formed 
between  the  two  valves,  did  not  the  lower  one  open  and  admit 
the  water  through  the  pipe  above  it.  The  lever  again  being 
worked,  the  same  process  is  repeated,  and  the  water  is  elevated 
to  the  spout  in  an  interrupted  stieam. 

The  tube,  with  the  stop-cock  C,  leading  from  the  barrel  to 
the  pipe,  is  added  for  the  purpose  of  letting  the  water  escape 
from  the  former  in  cold  weather,  and  thus  prevent  its  freezing. 

592.  Although,  in  common  language,  this  is  called  the  suc- 
tion pump,  still  it  will   be  observed  that  the  water  is  elevated 
by  suction,  or,  in  more  philosophical  terms,  by  atmospheric 
pressure,  only  above  the  valve  A,  after  which  it  is  raised  by  lift- 
ing up  to  the  spout.     The  water,  therefore,  is  pressed  into  the 
pump-barrel  by  the  atmosphere,  and  thrown  out  by  the  power 
of  the  lever. 

593.  LIFTING   PTJMP. — The 
lifting  pump,  properly  so  called, 
has  the  piston  in  the  lower  end 
of  the  barfll,  and  raises  the 
water  through  the  whole  dis- 
tance,   by  forcing  it   upward, 
without  the  agency  of  the  at- 
mosphere. 

In  the  suction  pump,  the 
pressure  of  the  atmosphere  will 
raise  the  water  33  or  34  feet, 
and  no  more,  after  which  it 
may  be  lifted  to  any  height  re- 
quired. 

594.  FORCING  PUMP. — The 
forcing  pump  differs  from  both 
these,  in  having  its  piston  solid, 
or  without  a  valve,  and  also  in 
having    a   side  pipe,  through 
which  the  water  is  forced,  in- 
stead of  rising  in  a  perpendicu- 
lar direction,  as  in  the  others.  Forcing  Pump. 

595.  The  forcing   pump  is 

represented  by  Fig.  134,  where  A  is  a  solid  piston,  working 

592.  How  far  is  the  water  raised  by  atmospheric  pressure,  and  how  far  by  lifting? 
593.  How  does  the  lifting  pump  differ  from  the  common  pump  ?  594.  How  does  the 
forcing  pump  differ  from  the  common  pump  ? 


162  WATER    PUMPS. 

air-tight  in  its  barrel.  The  tube,  C,  leads  from  the  barrel  to 
the  air-vessel,  D.  Through  the  pipe,  P,  the.  water  is  thrown 
into  the  open  air.  G  is  a  guage,  by  which  the  pressure  of  the. 
water  in  the  air-vessel  is  ascertained.  Through  the  pipe  I,  the 
water  ascends  into  the  barrel,  its  upper  end  being  furnished 
with  a  valve  opening  upward. 

To  explain  the  action  of  this  pump,  suppose  the  piston  to  be 
down  to  the  bottom  of  the  barrel,  and  then  to  be  raised  upward 
by  the  lever  L ;  the  tendency  to  form  a  vacuum  in  the  barrel, 
will  bring  the  water  up  through  the  pipe  I,  by  the  pressure  of 
the  atmosphere.  Then,  on  depressing  the  piston,  the  valve  at 
the  bottom  of  the  barrel  will  be  closed,  and  the  water,  not  find- 
ing admittance  through  the  pipe,  whence  it  came,  will  be  forced 
through  the  pipe  C,  and  opening  the  valve  at  its  upper  end, 
will  enter  into  the  air-vessel  D,  and  be  discharged  through  the 
pipe  P,  into  the  open  air. 

The  water  is  therefore  elevated  to  the  piston-barrel  by  the 
pressure  of  the  atmosphere,  and  afterward  thrown  out  by  the 
force  of  the  piston.  It  is  obvious,  that  by  this  arrangement, 
the  height  to  which  this  fluid  may  be  thrown,  wijl  depend  on 
the  power  applied  to  the  lever,  and  the  strength  witn  which  the 
pump  is  made. 

596.  The  air-vessel  D  contains  air  in  its  upper  part  only,  the 
lower  part,  as  we  have  already  seen,  being  filled  with  water. 
The  pipe  P,  called  the  discharging  pipe,  passes  down  into  the 
water,  so  that  the  air  can  not  escape.  The  air  is  therefore  com- 
pressed, as  the  water  is  forced  into  the  lower  part  of  the  vessel, 
and  reacting  upon  the  fluid  by  its  elasticity,  throws  it  out  of  the 
pipe  in  a  continued  stream.  The  constant  stream  which  is 
emitted  from  the  direction  pipe  of  the  fire-engine,  is  entirely 
owing  to  the  compression  and  elasticity  of  the  air  in  its  air-ves- 
sel. In  pumps,  without  such  a  vessel,  as  the  water  is  forced 
upward  only  while  the  piston  is  acting  upon  it,  there  must  be 
an  interruption  of  the  stream  while  the  piston  is  ascending,  as 
in  the  common  pump.  The  air-vessel  is  a  remedy  for  this  de- 
fect, and  is  found  also  to  render  the  labor  of  drawing  the  water 
more  easy,  because  the  force  with  which  the  air  in  the  vessel 
acts  on  the  water,  is  always  in  addition  to  that  given  by  the 
force  of  the  piston. 

595.  Explain  Fift.  134,  and  show  in  what  manner  the  water  is  brought  up  through 
the  pine  I,  and  afterward  thrown  out  at  the  pipe  P.  596.  Why  does  not  the  air  es- 
cape from  the  air-vessel  in  this  pump  1  What  effect  does  the  air-vessel  have  on  the 
stream  discharged  1  Why  does  the  air-vessel  render  the  labor  of  raising  the  water 
more  easy  1 


WATER    PUMPS. 


163 


FIG.  135. 


597.  ATMOSPHERIC  AND  FORCING  PUMP. — A  curious  com 
bi nation  of  the  atmospheric  and  forcing  pumps,  is  the  following 
Fig.  135. 

The  atmospheric,  is  furnished  with  a  rod 
and  piston,  with  the  valve  C,  opening  in  the 
usual  manner.  The  forcing  piston  B,  is  of 
solid  metal,  working  water-tight  in  its  bar- 
rel. The  barrels  are  joined  below  the  valve 
D,  their  pistons  being  also  connected  by  a 
cross-bar,  A,  between  the  rods,  so  that  they 
rise  and  fall  together. 

Now  when  the  lever  is  depressed,  and  the 
pistons  raised,  the  water  above  the  valve  C 
is  discharged  at  the  spout  in  the  manner  of 
the  common  suction  pump,  and  the  space 
is  filled  by  atmospheric  pressure  through 
the  lower  valve  D,  by  the  suction  pipe. 
When  the  pistons  descend,  this  valve  closes, 
and  the  solid  piston  B,  drives  the  water 
through  the  valve  C,  and  above  that  piston 
and  to  the  spout.  Thus  one  piston  operates 
when  the  lever  rises,  and  the  other  when  it 
falls,  producing  in  effect  a  constant  stream 
of  water  from  the  spout. 

In  the  construction  of  this  pump,  it  should 
be  considered  that  as  both  cylinders  are 
filled  at  the  same  time,  the  suction  pipe 
ought  to  be  large  in  proportion. 

598.  STOMACH  PUMP. — The   design  of 

this  pump,  of  which  there  are  several  varieties,  is  to  throw  a 
fluid  into  the  stomach,  and  again  to  withdraw  it  without  chang- 
ing the  apparatus,  but  only  its  position.  In  cases  of  poisoning, 
the  contents  of  the  stomach  may  thus  be  diluted  and  withdrawn, 
including  the  deleterious  matter,  and  thus  the  life  of  the  indi- 
vidual be  saved. 

599.  That  here  described  is  from  the  Journal  of  the  Franklin 
Institute*     It  consists  of  a  common  metallic  syringe,  A,  Fig. 
136,  screwed  to  a  cylindrical  valve-box,  B,  which  contains  two 
ovoid  cavities,  in  each  of  which  there  is  a  loose,  spherical  me- 
tallic valve.     The  ends  of  the  cavities  are  pierced,  and  the  valves 

597.  What  is  the  difference  between  the  pump,  ^isr.  135,  and  the  common  atmos. 
pheric  and  forcing  pump  7  598.  What  is  the  use  of  tne  stomach  pump  1  599.  De- 
scribe the  stomach  pump,  and  shgw  the  reason  why  it  acts  in  opposite  ways  on  being 
turned  over. 


Atm 


'spheric  and  Forc- 
ing Pump. 


164  WATER   PUMPS. 

FIG.  136. 


Stomach  Pump. 

fit  exactly,  either  of  the  orifices.  Thus  it  makes  no  difference 
which  end  of  the  valve-box  is  upturned,  the  valve  falling  down 
and  closing  the  orifices  in  either  direction.  The  flexible  India 
rubber  tubes,  C  D,  are  attached  to  the  opposite  ends  of  the 
cavities. 

Now  suppose  the  then  upper  tube  is  introduced  into  the 
stomach,  and  the  lower  one  into  a  basin  of  warm  water ;  in  this 
position,  on  working  the  syringe  the  liquid  would  be  injected 
into  the  stomach,  and  the  poison  diluted ;  then  on  reversing  the 
position,  by  turning  the  syringe  in  the  hand,  without  withdraw- 
ing the  tube  from  the  stomach,  the  valves  drop  on  the  other 
orifices,  and  the  water  is  pumped  from  the  stomach  into  the 
basin,  as  represented  by  the  figure. 

This  is  an  interesting  and  beautiful  invention",  and  no  doubt 
has  been  the  instrument  of  saving  many  human  lives  in  cases 
of  poisoning. 

600.  FIRE  ENGINE. — The  fire  engine  is  a  modification  of  the 
forcing  pump.  It  consists  of  two  such  pumps,  the  pistons  of 
which  are  moved  by  a  lever  with  equal  arms,  the  common  ful- 
crum being  at  C,  Fig.  137.  While  the  piston  A  is  descending, 
the  other  piston,  B,  is  ascending.  The  water  is  forced  by  the 
pressure  of  the  atmosphere,  through  the  common  pipe  P,  and 
then  dividing,  ascends  into  the  working  barrels  of  each  piston, 
where  the  valves,  on  both  sides,  prevent  its  return.  By  the 
alternate  depression  of  the  pistons,  it  is  then  forced  into  the  air- 
box  D,  and  then,  by  the  direction  pipe  E,  is  thrown  where  it  is 


600.  Explain  Fig.  137,  and  describe  the  action  of  the  fire  engine.    What  causes  the 
continued  stream  from  the  direction  pipe  of  this  engine  ? 


WATER   PUMPS. 


165 


FIG.  137. 


FIG.  13& 


Fire  Engine. 


wanted.  This  machine  acts  precisely  like  the  forcing  pump, 
only  that  its  power  is  doubled,  by  having  two  pistons  instead 
of  one. 

601.  ROTARY  PUMP.  —  This  is  an  ancient  invention,  though 
more  than  once  re-invented  and  constructed  in  various  forms  in 
modern  times.  That  here  represented,  Fig.  138,  according  to 
Mr.  Ewbank,  from  whom  the  cut  is  taken,  is  one  of  the  oldest, 
as  well  as  best,  ever  constructed. 

The  design  is  to  produce  a  continued  stream,  by  simply  turn- 
ing a  crank,  thus  converting  the  piston  into  cog-wheels,  and  the 
vertical  motion  into  a  rotary  one. 

Its  construction  is  as  follows  :  Two  metallic  cog-wheels,  with 
obtuse  teeth,  are  inclosed  in  a  metallic  case,  so  nicely  fitted  to 
each  other  that  the  water  can  not  escape  between  them.  The 
teeth  also  work  so  accurately  between  each  other  as  to  retain 
the  fluid.  The  axle  of  one  of  the  wheels  is  continued  through 
one  side  of  the  case  to  receive  the  crank  by  which  it  is  turned, 
the  joint  being  secured  by  a  collar  of  leather. 

One  side  of  the  case  being  removed  in  the  figure  to  show  the 
construction,  it  will  be  observed  that  the  motion  of  one  wheel. 


. 
by  the  figure. 


said  of  the  antiquity  of  the  rotary  pump  1    Explain  its  construction 
What  objection  to  this  pump  is  stated  ? 


166 


WATER    PUMPS. 


will  turn  the  other  in  the  opposite  direction,  the  arrows  show- 
ing the  course  of  the  water. 

Now  the  wheels  being  water-ti^ht  between  themselves  and 
both  sides  of  the  case,  the  only  vacant  spaces  for  the  water  are 
those  between  the  cogs,  as  they  revolve,  and  the  diameter  of 
the  case. 

The  machine  being  put  in  motion,  the  water  enters  the  case 
by  the  suction  pipe  B,  is  carried  up  by  the  cogs  in  succession, 
and  these  being  always  in  contact,  it  can  not  escape  except  at 
the  forcing  pipe  A,  where  it  issues  in  a  continued  stream.  This, 
therefore,  is  a  suction  and  forcing  pump  in  one. 

But  the  friction  is  such  between  the  metallic  surfaces  that  the 
machine  remains  perfect  only  for  a  short  time,  nor  does  it  ap- 
pear that  the  recent  improvements  in  this  sort  of  pump  have 
been  such  as  to  bring  it  into  general  use,  and  the  defects  of  the 
plan  seem  to  be  insuperable. 

602.  FOUNTAIN  OF  HIERO. — There  FIG.  139. 

is  a  beautiful  fountain,  called  the 
fountain  of  Hiero,  which  acts  by 
the  elasticity  of  the  air,  and  on  the 
principle  of  hydrostatic  pressure.  Its 
construction  will  be  understood  by 
Fig.  139,  but  its  form  may  be  varied 
according  to  the  dictates  of  fancy  or 
taste.  The  boxes  A  and  B,  together 
with  the  two  tubes,  are  made  air- 
tight, and  strong,  in  proportion  to 
the  height  it  is  desired  the  fountain 
should  play. 

To  prepare  the  fountain  for  action, 
fill  the  box  A  through  the  spouting 
tube,  nearly  full  of  water.  The  tube 
0,  reaching  nearly  to  the  top  of  the 
box,  will  prevent  the  water  from 
passing  downward,  while  the  spout- 
ing pipe  will  prevent  the  air  from  mero's  Fountain. 
escaping  upward,  after  the  vessel  is 

about  half  filled  with  water.  Next,  shut  the  stop-cook  of  the 
spouting  pipe,  and  pour  water  into  the  open  vessel  D.  This 
will  descend  into  the  vessel  B,  through  the  tube  E,  which  nearly 
reaches  its  bottom,  so  that  after  a  few  inches  of  water  are  poured 


602.  How  is  the  fountain  of  Hiero  constructed  ?    On  w^at  will  the  height  of  the  jet 
from  Hiero'g  fountain  depend  7 


DISTRIBUTION    OF    HEAT.  167 

in,  no  air  can  escape,  except  by  the  tube  C,  up  into  the  vessel 
A.  The  air  will  then  be  compressed  by  the  weight  of  the 
column  of  water  in  the  tube  E,  and  therefore  the  force  of  the 
water  from  the  jet-pipe  will  be  in  proportion  to  the  height  of 
this  tube.  If  this  tube  is  20  or  30  feet  high,  on  turning  the 
stop-cock,  a  jet  of  water  will  spout  from  the  pipe  that  will  amuse 
and  astonish  those  who  have  never  before  seen  such  an  experiment. 


CHAPTER    VIII. 

HEAT,  AND  THE   LAWS  OF  ITS  ACTION. 

603.  In  respect  to  the  laws  of  incidence  and  reflection,  and 
in  many  other  respects,  the  phenomena  of  light  and  heat  are  the 
same.     But  in  respect  to  transmission,  radiation,  distribution, 
effects  on  other  substances,  both  chemical  and  mechanical,  and 
the  manner  in  which  it  affects  our  senses,  there  are,  it  is  well 
known,  great  differences. 

DISTRIBUTION    OF   IIEAT. 

604.  The  rays  of  heat  falling  on  a  body  are  disposed  of  in 
three  ways.     First,  they  may  be  reflected,  or  rebound  from  the 
surface  ;  second,  they  may  be  absorbed  or  received  into  the  sub- 
stance of  the  body ;  or  third,  they  may  be  transmitted,  or  pass 
through  its  substance. 

605.  Reflection. — Radiant  heat,  that  is,  heat  flowing  from 
any  hot  body,  is  like  light  reflected  from  polished  surfaces,  and 
as  in  light,  the  angle  of  reflection  is  equal  to  that  of  incidence. 
Those  surfaces,  however,  which  reflect  light  most  perfectly,  are 
not  always  the  best  reflectors  of  heat.     Thus,  polished  metals 
are  the  best  reflectors  of  heat,  while  glass,  which  reflects  light 
most  perfectly,  is  a  very  imperfect  reflector  of  heat ;  thus  tin 
plate  reflects  about  eight  times  as  much  heat  as  a  glass  mirror. 

606.  Absorption. — Radiant  heat  is  absorbed  with  very  differ- 
ent facilities  by  bodies  and  surfaces  of  different  kinds.     Those 

603.  In  what  respects  are  action  of  heat  and  light  the  same  ?  In  -what  respects  are 
their  phenomena  dissimilar  ?  604.  In  what  ways  are  the  rays  of  light  diffused  1  605 
What  is  meant  by  rellection  of  heat  7  606.  What  by  absorption  ?  What  by  radia 
tion  1  What  surfaces  reflect  heat  best  7  Give  examples.  What  surfaces  possess  the 
greatest  absorbing  powers  7  Examples. 


168  DISTRIBUTION    OF    HEAT. 

surfaces  which  radiate  most  readily,  absorb  heat  with  the  great- 
est, facility,  and  on  the  contrary,  those  surfaces  which  radiate 
feebly,  do  not  readily  absorb  heat.  Thus  a  plate  of  tin,  if 
painted  black,  will  both  absorb,  and  radiate  perfectly ;  while,  if 
the  surface  retains  its  bright  metallic  polish,  it  will  neither  ab- 
sorb nor  radiate.  Hold  the  black  surface  near  the  fire,  and  the 
metal  will  soon  become  too  hot  for  the  fingers ;  while  the  bright 
surface  will  not  become  even  warm,  by  the  same  exposure 
There  is  also  a  difference  between  culinary  heat  and  that  of  the 
sun,  with  respect  to  absorption,  for  if  a  piece  of  plate  glass  be 
held  before  the  fire  it  soon  becomes  hot,  while  every  window 
shows  by  the  temperature  of  the  glass,  that  it  does  not  absorb 
the  heat  of  the  sun. 

607.  Transmission. — Most  transparent  substances  transmit 
heat,  that  is,  allow  it  to  pass  through  their  pores,  with  more  or 
less  facility;  in  this  respect,  however,  experiment  proves  that 
there  are  great  differencesjn  bodies,  where  from  external  ap- 
pearance, little  or  none  might  be  expected.     Thus,  rock-crystal 
transmits  heat  very  perfectly,  while  alum,  though  equally  trans- 
parent, admits  few  of  the  calorific  rays,  to  pass  through  it.     This  • 
difference  is  so  great,  that  a  piece  of  smoky,  brown  rock-crystal, 
which  was  fifty-eight  times  thicker  than  a  transparent  plate  of 
alum,  transmitted  19  rays,  while  the  alum  transmitted  only  6. 
The  cause  of  this  remarkable  difference  is  unknown,  though 
probably  it  depends  on  the  crystaline   structure  of  the  two 
substances. 

608.  Operation  of  these  Laws. — The  general  diffusion  of  heat 
seems  to  depend  on  the  operation  of  the  above  described  natural 
laws,  and  hence  it  is,  that  in  the  same  vicinity,  two  thermome- 
ters graduated  alike,  and  equally  exposed,  always  indicate  the 
same  temperature. 

609.  When  the  sun,  that  universal  source  of  heat,  as  well  as 
of  light,  radiates  his  rays  upon  the  earth,  they  are  absorbed  by 
some  bodies,  and  transmitted  or  reflected  by  others,  according 
to  their  several  powers,  or  natures.     But  the  great  means  of  the 
general  and  equal  diffusion  of  heat,  is  the  earth  itself,  arid  the 
atmosphere  with  which  it  is  surrounded.     Having  absorbed  the 
radiant  heat  of  the  sun,  the  ground  becomes  in  its  turn,  a  radiant 
source  to  all  surrounding  objects,  while  the  atmosphere  acts  as 
a  perpetual  absorbent,  rising  up  from  the  earth,  in  proportion 


607.  Give  examples  of  the  transmission  of  heat  through  substances.  608.  What  ar? 
the  means  of  the  general  diffusion  of  heat  1  609.  By  what  means  is  it  said  this  law  is 
illustrated  in  rooms? 


THERMOMETER.  169 

*o  the  quantity  of  heat  it  obtains,  and  again  sinking  down,  in 
cooler  places.  Thus  there  is  a  constant  interchange  among  the 
warmer,  and  cooler  strata  of  the  atmosphere,  while  currents  in 
the  form  of  wind,  tend  to  mix  these  with  each  other,  making 
the  temperature,  at  the  same  distance  from  the  earth  and  in  the 
sane  vicinity  every  where  the  same.  This  law  of  equal  distri- 
bution is  strikingly  illustrated  in  rooms  warmed  by  the  admis- 
sion of  hot  air  from  beneath,  for  although  the  register,  or  place 
of  admission  may  be  in  one  corner,  or  through  the  partition,  still 
the  temperature  m  in  every  part  of  the  room,  with  the  exception 
of  over  the  register,  is  the  same.  Even  rooms,  30  or  40  feet  iu 
length,  and  when  the  air  is  admitted  through  only  one  register, 
and  this  in  a  corner,  are  made  equally  comfortable  throughout, 
by  this  admirable  method. 

THERMOMETER. 

610.  Did  not  the  heat  diffuse  itself  as  above  described,  the 
thermometer  would  be  entirely  itseless,  since  several  in  the  same 
vicinity,  though  graduated  exactly  alike,  would  indicate  different 
temperatures. 

611.  The  term  thermometer  comes  from  two  Greek  words, 
signifying  heat  measurer  ;  and  its  use  strictly  corresponds  to  the 
name,  being  an  instrument  for  comparing  the  degrees  of  free 
heat  existing  in  other  bodies.     This  it  does  by  the  expansion 
and  contraction  of  a  fine  thread  of  mercury,  confined  in  a  glass 
tube,  having  a  small  reservoir  of  the  same  metal  at  the  lower 
end,  called  the  bulb.   ' 

612.  Mercury  is  employed  for  this  purpose  for  several  rea- 
sons ;  one  is  that  fluids,  as  alcohol,  occupy  too  much  space  ; 
another,  that  this  metal  is  more  uniform  in  expanding  and  con- 
tracting than  any  other  substance ;  and  lastly,  it  is  not  liable  to 
vaporize  in  the  vacuum  in  which  it  is  placed,  and  thus,  like 
liquids,  to  interfere  with  its  own  variation  in  the  stem. 

613.  ALCOHOLIC    THERMOMETER. — Although    mercury,    or 
quicksilver,  is  the  best  substance  known  for  the  construction  of 
thermometers,  and  is  that  universally  employed  in  temperate 
climates,  yet  it  is  objectionable  in  extreme,  or  polar  latitudes, 
on  account  of  its  liability  to  freeze.     In  Siberia,  and  other  north- 
ern inhabited  regons,  where  the  cold  is  often  down  to  — 40°  of 
Fahrenheit's  scale,  alcoholic  thermometers  are  of  necessity  em- 

610.  What  issaid  of  the  use  of  the  thermometer  without  an  equal  diffusion  of  h  at  T 
611.  What  does  thermometer  mean  ?  612.  Why  is  mercury  used  in  thermometers  "n 
preference  to  liquids  ?  613.  Why  are  alcoholic  thermometers  used  1 

8 


170  THERMOMETER. 

ployed,  since  at  that  point  mercury  becomes  solid  by  freezing, 
and  therefore  useless.  These  thermometer  tubes  are  much 
longer  than  ordinary,  since  alcohol  expands  in  a  greater  propor- 
tion than  mercury  by  the  same  increment  of  heat. 

614.  Different  Mercurial  Thermometers. — There  are  three 
thermometers  in  general  use,  namely,  Fahrenheit^,  which  is 
ir^ed  in  England,  and  in  this   country  ;  the   Centigrade,  con- 

tructed  by  Celsius,  which  is  generally  used  in  France ;  and 
Reaumur's  thermometer,  adopted  in  Germany.' 

615.  Fahrenheit,  (Fah.) — In  this  the  intermediate  space  be- 
tween the  freezing  and  boiling  points  is  divided  into  180  de- 
grees;  the  freezing  being  marked  32°,  and  the  boiling  212°. 
This  scale  was  invented  by  Fahrenheit,  from  an  erroneous  belief 
that  32  of  these  divisions  below  the  freezing  point  of  water, 
which  is  therefore  0  on  -the  scale,  indicated  the  zero,  or  greatest 
degree  of  cold.     But  he  afterward  discovered  his  error,  and  his 
instrument  being  in  use,  corrected  it  as  far  as  possible,  by  add- 
ing a  series  of  descending  degrees  below  his  zero,  prefixing  to 
them  the  sign  — ,  or  minus,  that  is,  below  zero. 

•  616.  Centigrade,  (Cent.) — It  is  also  sometimes  indicated  by 
Cel,  for  the  name  of  the  inventor.  It  consists  of  an  arrange- 
ment of  the  scale,  in  which  the  freezing  point  is  marked  0,  or 
zero,  and  the  boiling  point  is  marked  100°.  This  is  a  more 
convenient  scale  than  the  other,  the  freezing  and  boiling  points 
being  even  numbers,  and  all  below  the  former  —  minus. 

617.  Reaumur,  (Beau.) — In  this  the  freezing  point,  as  in 
the  laSt,  is  marked  0,  while  the  boiling  f>oint,  instead  of  being 
100°,  is  marked  80°.     The  degrees  are  continued  both  above 
and  below  these  points,  those  below  being  negative  or  minus,  • 
as  in  the  others. 

These  Thermometers  Compared. — In  books  of  foreign  travels, 
where  the  author  adopts  the  thermometer  of  the  country  he  de- 
scribes, the  reader  is  often  perplexed  to  know  what  degrees  of 
temperature  are  indicated  according  to  his  own  scale,  by  what 
he  reads.  Figures  are  therefore  added  of  each,  Fig.  140,  to- 
gether with  a  table  showing  the  correspondence  of  the  three, 
and  the  rules  for  converting  one  scale  into  the  others. 

618.  Thus  the  Centigrade  scale  is  reduced  to  that  of  Fahren- 
heit, by  multiplying  by  9  and  dividing  by  5,  and  that  of  Reau- 


614  What  are  the  names  of  the  mercurial  thermometers?  615.  What  are  the 
div  sions  of  Fahrenheit's  scale?  616.  What  are  those  of  the  Centigrade  1  617.  Wha' 
are  those  of  Reaumur  7  618.  Ho  v  is  the  Centigrade  reduced  to  that  of  Fahrenheit  t 
Itow  is  that  of  Reaumur  reduced  'o  that  of  Fahrenheit  7 


THERMOMETER 
FIG.  140. 


171 


Fahrenheit. 


Centigrade. 


Reaumur. 


mur  to  that  of  Fahrenheit,  by  multiplying  by  9  and  dividing  by 
4 ;  or  that  of  Fahrenheit  to  either  of  the  others  by  reversing 
these  processes.  Examples  : — 

Cent.  100°  X    9  =  900-4-5  =  180+32  =  212°  Fah. 
Reau.     80-' X    9  =  720-4-4  =  180  +  32  =  212°  Fah. 
Fah.     212°  — 32  =  180x5  =  900-4-   9  =  100°  Cent. 
Fah.     212°  — 32  =  180  X4  =  720^-   9=   80°  Reau. 

The  following  Table  from  Prof.  Hoblyn's  Dictionary  of  Science, 
shows  at  a  single  view  the  correspondence  between  these  ther- 
mometers, from  the  zero  to  the  boiling  point  of  Fahrenheit. 

Fahrenheit.                           Centigrade.  Reaumur. 

BOILING.       212 100  80 

200 93.33 74.66 

190 87.77 70.22 

180 82.22 65.77 

170 7G.C6 61.33 

160 71.11 56.88 

150 65.55 52.33 

140 60    48 

130 55.55 33.55 

120  .  48.88  .         .  39.11 


172 


THERMOMETER. 


Fahrenheit.                           Centigrade. 

110  43.33  . 
100  37.77  . 
90  32.22  . 
80  26.^>fi 

Reaumur. 
.      .      .      .34.66 
....   30.22 
....   25.77 
21  33 

FREEZING* 
ZERO. 

70  

21*11 

16  88 

60  
50  

15.55  . 
10 

....  12.44 
.     .                8 

40  

4  44 

3  35 

32  

0 

.     .     .     .     0 

20  

6  66  . 

5  33 

10  

0  . 

12.22  . 

17.77  . 

.     .     .     .     9.77 

.  14.22 

619.  Rutherford's  Register  Thermometer. — By  this,  the  high- 
est and  lowest  temperatures  which  occur  within  a  given  time 
are  indicated,  and  made  to  register  themselves.  This  instru- 
ment consists  of  two  thermometers  fastened  to  the  same  plate 
with  their  tubes  in  a  horizontal  position,  as  shown  by  Fig.  141. 


FIG.  141. 


Rutherford's  Register  Thermometer. 

One  of  these,  A,  contains  alcohol ;  the  other,  B,  contains  mer- 
cury. In  the  stem  of  B,  a  small  piece  of  iron  wire  acts  the  part 
of  an  index,  being  propelled  forward  as  the  mercury  expands, 
and  being  left  at  the  point  of  the  greatest  expansion  when  the 
mercury  contracts,  thus  indicating  the  highest  temperature  to 
which  it  had  been  exposed.  In  the  stem  of  the  other,  a  small 
piece  of  ivory,  A,  is  immersed  in  the  alcohol,  and  by  a  slight 
inclination  of  the  instrument,  is  brought  to  the  surface  of  the 
liquid.  When  the  temperature  falls,  the  ivory,  by  adhering  to 
the  liquid,  is  drawn  back  with  it ;  but  when  it  rises,  the  spirit 
only  advances,  leaving  the  ivory  behind,  thus  indicating  the 
lowest  temperature  which  had  occurred  since  the  last  observa- 

619.  What  are  the  indications  made  by  Rutherford's  thermometer?  Describe  the 
construction  of  this  instrument.  What  are  the  peculiar  advantages  of  this  in 
strument  1 


HYGROMETER. 


173 


1FIG-  142- 


tion.  By  inverting  the  instrument,  the  particle  of  ivory  is  again 
brought  to  its  place  for  a  new  observation.  This  is  a  very  con- 
venient thermometer  on  many  accounts.  Thus  the  highest 
temperature  during  the  day  or  the  week,  can  be  told  without 
watching  the  instrument,  and  at  a  single  inspections  If  it  is  re- 
quired to  obtain  the  degree  of  heat  at  the  bottom  of  a  deep 
well,  or  in  the  depths  of  the  sea,  this  can  be  done  accurately  by 
letting  down  the  instrument,  while  the  common  thermometer 
would  change  while  drawing  it  up. 

620.  DIFFERENTIAL  THERMOMETER.  — 
This  instrument   is  shown  by  Fig.  142. 
It  consists  of  two  thin  glass  bulbs  of  an 
inch  in  diameter,  contacted  by  a  glass 
tube  bent  at  right  angles,  as  the  figure 
shows.     This  tube  is  partly  filled  with  col- 
ored alcohol.     Now  when  one  of  the  bulbs 
is  heated  more  than  the  other,  the  air  in 
it  expands,  and  drives  the  liquid  into  the 
other  bulb. 

621.  It  does  not,  therefore,  indicate  the 
temperature  of  the  atmosphere,   as  the 

same  degree  of  heat  on  both  bulbs  at  the  same  time  produces  no 
change,  its  design  being  merely  to  show  the  difference  of  tem- 
perature to  which  the  bulbs  are  exposed. 

HYGROMETER.  FIG.  143. 

622.  The  name  of  this  instrument, 
from    the    Greek,   signifies    "  moisture 
measurer."     Its  use  is,  to  show  the  state 
of  moisture  in  the  atmosphere.     Many 
inventions  for  this  purpose  have  been 
tried,  but  that  represented  by  Fig.  143, 
is  at  present  considered  the  best. 

It  is  called  DanieVs  dew-point  hygrorz- 
eter.  It  consists  of  two  balls,  connected 
together  by  a  bent  tube,  as  shown  by  the 
figure,  the  whole  being  of  glass.  The 
ball  B,  contains  a  small  quantity  of 
ether,  by  the  boiling  of  which,  the  air 
has  been  expelled  from  the  tube.  In  it 
a  small  thermometer  is  placed,  with  its 
bulb  in  the  ball.  The  lower  part  of  this 
ball  is  gilded,  that  the  deposited  dew  __  - 

may  be  visible.     The  other  ball,  A,  is  Hygrometer. 


174  STEAM   ENGINE. 

covered  with  muslin,  and  is  kept  moist  with  ether,  the  evapora 
tion  of  which  produces  cold,  which  gradually,  by  the  evaporation 
of  the  ether  in  the  other  ball,  reduces  the  temperature  in  that 
to  the  dew-point,  which  is  indicated  by  the  deposition  of  moisture 
on  the  gilded  ball. 

The  degree  of  temperature  at  which  this  deposition  takes 
place,  is  shown  by  the  thermometer  in  the  tube,  and  this  degree 
is  called  the  dew-point,  and  this  is  effected  at  a  higher  or  lower 
degree,  according  to  the  moisture  in  the  atmosphere.  The  ther- 
mometer on  the  stem  indicates  the  temperature  of  the  air  at  tha 
time  when  the  observations  are  made. 


CHAPTER  IX. 

STEAM    ENGINE. 

NOTE. — The  following  description  of  the  steam  engine  is 
taken  from  Prof.  Hoblyn's  Edition  of  the  Author's  Natural 
Philosophy,  published  by  Adam  Scott,  Charter-house  Square, 
London. 

We  have  however  omitted,  in  this  edition,  the  ingenious  ma- 
chines of  Hero,  Branca,  and  Savery,  contained  in  former  copies, 
as  merely  showing  the  progress  of  invention,  and  being  quite 
unnecessary  for  the  comprehension  of  the  engine,  as  it  exists 
at  the  present  day.  This  omission  will  be  found  replaced  by 
some  of  the  most  important  inventions  of  the  present  day. 

The  description  of  Newcomen's  engine  has  been  retained,  as 
containing  some  parts,  leading  to  the  explanation  of  Watt's  en- 
gine, by  which  it  was  succeeded. 

What  is  meant  by  the  double-action  of  Watt's  engine,  con- 
sisted in  the  application  of  steam  alternately  on  each  side  of  the 
piston,  and  by  which  it  was  moved  both  up  and  down,  while 
that  of  Newcomen  was  moved  only  in  one  direction  by  the 

620.  What  is  the  construction  of  the  differential  thermometer?  621.  What  is  the 
use  of  this  instrument  ?  622.  What  is  the  meaning  of  the  term  hygrometer  (  What 
B  that  here  described  called  1  Explain  its  principle,  and  the  manner  of  using  it. 


BTEAM    ENGINE.  175 

steam,  and  in  the  other  by  the  pressure  of  the  atmosphere  over 
a  vacuum.  The  importance  of  Watt's  invention  can  hardly  be 
appreciated,  since  on  it  is  founded  the  action  of  all  steam  engines 
to  this  day. 

623.  NEWCOMEN'S  ATMOSPHERIC  ENGINE. — The  drainage  of 
deep  mines  was  a  matter  of  great  importance,  and  the  failure 
of  Silvery 's  engine  in  this  respect,  paved  the  way  to  further  ex- 
periment.    In  1705,  Thomas  Newcomen,  a  smith  of  Dartmouth, 
obtained  letters  patent  for  the  construction  of  a  new  kind  of 
steam  engine,  in  which  he  availed  himself  of  the  atmospheric 
pressure  in  a  different  way  from  that  adopted  by  Savery. 

624.  The  novelty  of  this  plan  consists  in  the  admission  of 
steam  beneath  an  air-tight  piston,  and  the  condensation  of  the 
steam  by  the  injection  of  cold  water  into  the  interior  of  the  cyl- 
inder.    The  use  of  a  cylinder  and  piston  may  be  easily  ex- 
plained.    In  order  that  the  pressure  of  steam  may  be  rendered 
available  in  machinery,  the  steam  must  be  confined  within  an 
air-tight  cavity,  so  constructed  that  its  dimensions,  or  capacity, 
may  be  altered  without  altering  its  tightness.     When  the  steam 
enters  such  a  vessel,  it  enlarges  its  actual  cavity,  by  causing 
some  movable  part  to  recede  before  it,  and  from  this  movable 
part  motion  is  communicated  to  machinery.     A  hollow  cylinder, 
having  a  movable  piston  accurately  fitted  to  its  bore,  constitutes 
a  vessel  of  this  kind ;  the  piston,  thus  employed,  has  an  alternate 
or  reciprocating  vertical  motion,  which  may  be  converted  into 
a  circular  motion  by  appropriate  machinery.     The  engine  em- 
ployed by  Newcomen,  in  its  most  improved  state,  was  as  fol- 
lows.    Over  a  boiler  a  is  fixed  a  cylinder  c,  containing  a  piston 
r,  the  rod  of  which  is  connected  with  one  of  the  arched  extrem- 
ities of  a  lever-beam  working  on  a  pivot ;  to  the  other  extremity 
of  the  beam  is  attached  a  chain  connected  with  the  pump-rod. 

625.  Such  is  the  simple  outline  of  the  atmospheric  engine. 
Its  mode  of  operation  is  as  follows :  Steam  is  admitted  from 
the  boiler  into  the  cylinder,  through  the  tube  I,  by  means  of  a 
regulating  cock,  e,  which  is  worked  by  a  handle  outside  the 
boiler ;  the  pressure  of  the  atmosphere  above  the  piston  being 
thus  balanced  by  the  force  of  the  steam  beneath  it,  the  extremity 
of  the  lever-beam  to  which  the  piston  is  attached  is  elevated  by 
proportionate  weights,  w,  attached  to  the  purnp-rod,  and  the 
piston  is  drawn  to  the  top  of  the  cylinder,  the  other  extremity 
of  the  beam  being  depressed. 

623.  What  was  Newcomen's  engine  called  1  624.  What  is  said  to  have  been  the 
novelty  of  Newcomeivs  plan  7  How  can  the  cavity  of  a  vessel  be  enlarged  by  steam 
and  still  be  tight?  625.  Describe  this  machine  by  the  figure. 


176 


STEAM   ENGINE. 
FIG.  144. 


Newcomen's  Engine, 

626.  In  order  to  effect  the  descent  of  the  piston,  the  steam  in 
the  cylinder  must  now  be  condensed.     The  regulating  cock  e  is 
accordingly  closed,  and  the  further  admission  of  steam  pre- 
vented ;  another  cock,  called  the  condensing  cock,  p,  is  now 
opened,  and  a  jet  of  cold  water  is  admitted  through  a  tube  from 
the  cistern  m,  which  is  placed  at  a  sufficient  height  to  insure  a 
forcible  injection ;  the  steam  in  the  cylinder  is  instantly  con- 
densed, a  vacuum  is  formed,  and  the  pressure  of  the  atmosphere 
forces  the  piston  to  the  bottom  of  the  cylinder,  while  the  pump- 
rod  on  the  other  end  of  the  beam  is  raised.     Such  is  the  gc'i- 
eral  operation  of  Newcomen's  atmospheric   engine,  which  is 
merely  a  pump  worked  by  steam. 

627.  WATT'S  DOUBLE-ACTING  ENGINE. — In  considering  the 
applicability  of  the  steam  engine  to  manufactures  generally,  it 

I   626.  Uow  was  the  steam  condensed  1    627.  What  was  Watt's  great  improvement 
in  the  steam  engine  ? 


STEAM    ENGINE.  IVY 

occurred  to  Watt,  that  if  lie  could  contrive  to  admit  steam 
alternately  above  and  below  the  piston,  and,  at  the  same  time, 
produce  a  vacuum  alternately  below  and  above  the  piston,  a 
double-acting  cylinder  would  be  produced,  an  impulse  thus  be 
communicated  by  the  ascent,  as  well  as  by  the  descent  of  the 
piston,  and  a  uniform  continuous  action  be  effected.  It  was  de- 
sirable, also,  to  convert  this  reciprocating  action  into  a  circulai 
one. 

628.  On  this  subject  "Watt  observes:  "Baring  made  my 
single  reciprocating  engines  very  regular  in  their  movements,  I 
considered  how  to  produce  rotative  motions  from  them  in  the 
best  manner;   and  among  various  schemes  which    were  sub- 
jected to  trial,  or  which  passed  through  my  mind,  none  appeared 
so  likely  to  answer  the  purpose  as  the  application  of  the  crank, 
in  the  manner  of  the  common  turning  lathe  ;  but  as  the  rota- 
tive motion  is  produced  in  that  machine  by  impulse  given  to 
the  crank  in  the  descent  of  the  foot  only,  it  requires  to  be  con- 
tinued in  its  ascent  by  the  energy  of  the  wheel,  which  acts  as  a 

fly- 

629.  "Being  unwilling  to  load  my  engine  with  a  fly-wheel 
heavy  enough  to  continue  the  motion  during  the  ascent  of  the 
piston  (or  with  a  fly-wheel  heavy  enough  to  equalize  the  mo- 
tion, even  if  a  counter-weight  were  employed  to  act  during  the 
ascent,)  I  proposed  to  employ  two  engines,  acting  upon  two 
cranks  fixed  on  the  same  axis,  at  an  angle  of  120°  to  one  an- 
other, and  a  weight  placed  upon  the  circumference  of  the  fly- 
wheel at  the  same  angle  to  each  of  the  cranks,  by  which  means 
the  motion  might  be  rendered  nearly  equal,  and  only  a  very- 
light  fly-wheel  would  be  requisite."     In  following  out  this  plan, 
some  very  important  changes  were  introduced  into  the   ma- 
chinery of  the  steam  engine:  the  principal  of  these  are  the 
double-acting  cylinder,  the  parallel  motion,  the  crank,  the  fly- 
wheel, and  the  governor.     Each  of  these  will  first  be  severally 

(  described ;  and  their  operation  in  the  double-acting  engine  be 
afterward  pointed  out. 

630.  Double-acting  Cylinder. — The  first  alteration  to  be  no- 
ticed in  the  double-acting  engine  is  that  of  the  cylinder.     To 
insure  its  double  action,  it  is  necessary  to  provide,  at  each  end 
of  the  cylinder,  a  means  of  admission  of  steam  from  the  boiler, 
and  of  escape  for  the  steam   to  the  condenser.     Hence  the 
double  action,  which  means  that  the  piston  is  both  raised  and 
depressed  by  the  force  of  steam. 

030.  What  is  meant  bv  the  double-acting  cylinder  ? 

"8* 


178 


STEAM    ENGINE. 


Double-acting 

Cylinder. 


631.  For  this  purpose,  a  steam-box  is  fixed 
to  eacli  end  of  the  cylinder,  communicating,  in 
the  one  case  with  the  upper,  in  the  other  with 
the  lower,  surface  of  the  piston.     In  Fig.  145, 
B  is  the  upper,  and  B'  the  lower,  steam-box. 
Each  of  these  boxes  is  furnished  with  two 
valves. 

632.  I.  In  the  upper  steam-box,  the  up- 
per, or  steam  vaWe,  S,  admits  steam  from  the 
boiler  through  a  tube,  the  mouth  of  which  is 
seen  immediately  above  the  valve  ;  the  lower, 
or  exhausting  valve,  C,  permits  the  escape  of 
the  steam  from  the  cylinder  to  the  condenser, 

through  a  tube  opening  immediately  below  the  valve.  In  this 
figure,  the  piston  is  at  the  top  of  the  cylinder ;  the  exhausting 
valve  is  therefore  represented  as  closed,  and  the  steam  valve  as 
open,  for  the  admission  of  steam,  which  rushes  through  the 
passage  D  to  the  top  of  the  cylinder,  in  order  to  force  the  piston 
to  the  bottom. 

633.  II.  In  the  lower  steam-box,  a  corresponding  mechan- 
ism is  observed,  and  its  valves  must  be  worked  at  the  same  mo- 
ment as  those  of  the  upper  box,  but  upon  an  exactly  opposite 
principle.     The  cylinder  is  full  of  steam,  and  the  piston  at  the 
top ;  the  steam  valve  S'  must  therefore  be  closed,  and  the  ex- 
hausting valve  C'  opened,  in  order  that  the  steam  may  rush 
out  at  the  passage  D',  and  a  vacuum  be  formed  beneath  the  pis- 
ton, to  give  effect  to  the  steam  which  is  now  entering  above  it. 

634.  In  Fig.  146,  the  piston  is  at  the  bot- 
tom of  the  cylinder.  1.  In  the  upper  steam- 
box,  the  steam  valve  S  is  accordingly  closed, 
and  the  exhausting  valve  C  opened,  to  admit 
of  the  escape  of  the  steam  from  above  the 
cylinder  through  the  passage  D  into  the  con- 
denser, and  thus  to  produce  a  vacuum  above 
the  piston.     2.  In  the  lower  'steam-box,  the 
exhausting  valve  C'  is  closed,  and  the  steam 
valve  S  opened,  in  order  that  steam  may  rush 
in  by  the  passage  D',  and  force  the  piston  to 
the  top  of  the  cylinder. 

From  the  preceding  description,  it  is  evi- 
dent that  the  alternate  motions  of  the  piston  depend  on  the 
opening  and  closing  of  the  valves,  alternately,  in  pairs.     When 


FIG.  146. 


Double-acting 
Cylinder. 


631.  Explain  the  double-acting  cylinder  by  Figs.  145  and  146. 


STEAM    ENGINE. 


179 


the  piston  is  at  the  top  of  the  cylinder,  the  upper  steam  valve 
and  the  lower  exhausting  valve  are  to  be  opened,  while  the 
lower  steam  valve  and  the  upper  exhausting  valve  are  to  be 
closed.  When  the  piston  is  at  the  bottom  of  the  cylinder,  this 
process  is  reversed. 

635.  PARALLEL  MOTION. — In  the  double-acting  engine,  the 
pressure  of  the  steam  acts  alternately  on  both  sides  of  the  pis- 
ton, which  must  therefore  be  pushed  upward  as  well  as  pulled 
downward;  the  connection  between  the  piston-rod  and  the 
beam  by  any  flexible  medium  is,  therefore,  obviously  inadmissi- 
ble ;  a  chain  can  not  communicate  an  upward  impulse  from  the 
piston  to  the  beam. 

The  difficulty  was,  to  adjust  the  rectilinear  motion  of  the  pis- 
ton-rod to  the  circular  motion  of  the  beam ;  without  such  ad- 
justment, it  is  evident  that  either  the  piston-rod,  being  forced 
to  the  right  and  left  alternately,  at  each  motion  of  ascent  and 
of  descent,  would  be  broken  or  bent ;  or  that  the  stuffing-box 
would  be  so  injured  by  these  derangements  of  action,  as  to  cease 


FIG.  147. 


Parallel  Motion. 

to  be  air  and  steam-tight.  The  contrivance  by  which  these 
difficulties  were  removed  by  Watt,  is  one  of  the  most  happy 
inventions  ever  introduced  into  machinery.  It  has  been  termed 
the  parallel  motion  ;  and  its  mechanism  may  be  understood  by 
means  of  the  subjoined  figure,  where  B  represents  the  end  of 

635.  Explain  by  Fig.  147,  how  parallel  motion  is  effected. 


180  STEAM    ENGINE. 

the  beam,  which  is  pulled  downward,  and  pushed  upward,  by 
the  motion  of  the  piston-rod  R  P ;  the  motion  of  B  is  in  the 
direction  of  the  dotted  curve  ;  that  of  R  P  is  rectilinear. 

636.  To  adjust  these  counteracting  motions,  a  series  of  bars 
are  introduced,  which  are  movable  on  pivots,  and  which  by  the 
balance  of  their  action  prevent  the  piston  from  deviating  to  any 
injurious  extent  from  the  straight  line.     Two  fixed  points  of 
support  are  taken,  the  one  at  F,  as  near  as  possible  to  the  line 
in  which  the  piston-rod  moves ;  the  other  at  C,  the  center  of 
the  working  beam.      Two  perpendicular  bars,  B  R  and  E  II, 
are  attached  to  the  beam  at  B  and  E ;  and  two  transverse  bars, 
R  H  and  F  H,  are  added,  the  former  connecting  the  lower  ex- 
tremities of  the  two  vertical  bars,  the  latter  connecting  the  lower 
extremity  of  the  vertical  bar  E  H  with  the  fixed  point  F ;  all 
the  bars  move  freely  on  pivots  at  all  their  points  of  attachment. 
The  head  of  the  piston-rod  is  connected  with  the  pivot  at  R. 
The  smaller  diagram,  Fig.  147,  relates  to  paragraph  639. 

637.  The  action  of  this  machinery  is  as  follows  :   1.  Let  us 
imagine  the  end  of  the  beam  B  to  descend  in  the  direction  of 
the  dotted  curve.     During  its  progress  to  the  horizontal  posi- 
tion, indicated  by  the  dotted  line  k  C,  it  is  continually  pushing 
the  perpendicular  bar  B  R  outward ;  and  this  effect,  if  not  coun- 
teracted, would  disturb  the  rectilinear  course  of  the  piston-rod. 
But  this  outward  push  of  the  bar  B  R  is  counteracted  by  an 
inward  pull  by  the  rod  R  II  upon  the  point  R ;  the  end  II  of 
the  rod    R    II  is    preserved   at   a  proper  distance   from    the 
line  of  motion  of  the  piston-rod  by  means  of  the  rod  called 
the  radius  rod,  H  F,  which   is   attached  to  the  fixed   point 
F;  and  the  rod  H  F,  being  thus  fixed,  describes,  with  its  ex- 
tremity H,  the  curve  d  a,  which  is  directed  inwardly,  and  coun- 
teracts the  outward  direction    of  the  curve   described  by  B. 
Hence  it  follows,  that  the  top  of  the  piston-rod  R  moves  in  a 
direction  almost  vertical.     It  is  correct  to  say  almost,  for  it  is 
not  strictly  so ;  the  deviation,  however,  from  the  vertical  motion 
involves  a  minute  calculation,  and  it  is  of  comparatively  little 
importance  in  practical  operation. 

638.  2.  As  the  beam  quits  the  horizontal  position  in  completing 
its  descent,  it  is  continually  pushing  the  bar  B  R  inward  ;  tut 
this  inward  push  of  ;the  bar  B  R  is  now  counteracted  by  the 
outward  pull  of  the  bar  H  F,  which  now  completes  the  curve 
g  o,  and,  by  means  of  the  transverse  connecting  bar  II  R,  main- 
tains the  piston-rod  in  its  nearly  vertical  direction.     3.  It  is  ob- 

636.  Explain  Watt's  engine  by  means  of  Fig.  117. 


STEAM    ENGINE.  181 

vious,  that  during  the  ascent  of  the  beam,  the  same  movements 
of  the  bars  will  secure  the  vertical  ascent  of  the  piston-rod. 
This  beautiful  contrivance  represents,  in  fact,  a  kind  of  jointed 
parallelogram,  three  of  the  angles  of  which  describe  curves, 
while  the  fourth,  which  is  connected  with  the  piston-rod,  moves 
nearly  in  a  straight  line. 

639.  MOTION  OF  THE  AIR-PUMP  ROD. — The  same  machinery 
which  regulates  the  motion  of  the  piston-rod  of  the  cylinder, 
also  regulates  those  of  the  pump-rod.     In  the  preceding  Fig. 
147,  the  upper  part  of  the  air-pump  rod  is  represented  at  A 
K ;  it  is  connected  at  the  top  to  the  middle  of  the  bar  E  H, 
where  it  works  freely  on  a  pivot  A.     This  machinery  may  be 
readily  understood  by  means  of  the  smaller  figure,  in  which  the 
bars  composing  it  are  separated  from  the  beam,  the  letters  be- 
ing preserved  precisely  as  in  Fig.  147.     C  E  and  F  H  are  two 
bars,  working  on  pivots  at  the  fixed  points  C  and  F,  and  de- 
scribing curves  at  their  free  extremities.     The  bar  E  H  con- 
nects these  free  extremities,  upon  which  it  moves  by  pivots. 
From  the  antagonizing  action  of  the  two  transverse  bars,  it  fol- 
lows, that  the  point  A,  the  head  of  the  air-pump  rod,  will  move 
in  a  nearly  vertical  direction. 

640.  NATURE  OF  THE  CRANK. — It  has  been  shown  that  the 
alternate  motions  of  the  piston-rod,  determined  by  the  double- 
acting  cylinder,  are  communicated  to  the  working  end  of  the 
beam,  to  the  curved  motion  of  which  they  are  adjusted  by  the 
contrivance  of  the  parallel  motion.     The  next  object  was  to  con- 
vert the  rectilinear  motion,   thus    produced,  into    a   rotatory 
motion. 

641.  So  long  as  the  force  of  steam  was  employed  for  the 
mere  purpose  of  raising  water,  no  such  motion  was  wanted ; 
but  when  its  application  was  required  for  the  purpose  of  turn- 
ing the  wheels  of  mills — of  giving  effect  to  the  machinery  of 
cotton  manufactures  and  printing  presses — of  propelling-  steam 
vessels  and  other  locomotive  engines — it  became  necessary  to 
impart  a  new  direction  to  its  operation.     To  obtain  this  object 
the  crank  was  introduced. 

642.  The  simplest  idea  of  a  crank  is  that  of  the  handle  to  a 
wheel;  its. action  is  familiarly  illustrated  in  the  process  of  draw- 
ing water  from  a  well ;  the  bent  handle  attached  to  the  wheel 
is  first  pushed  out,  then  pulled  in  the  opposite  direction,  and 
thus  a  continued  rotatory  motion  is  produced  upon  an  axle. 

639.  How  is  the  motion  of  the  air-pump  effected  ?    640.  What  is  the  cranic,  and 
how  does  it  act  1 


182 


STEAM    ENGINE. 


FIG.  148. 


The  application  of  this  principle  to  the  steam  engine,  and  the 
variations  of  pressure  on  the  crank  of  a  steam  engine,  may  be 
conveniently  illustrated  by  curves. 

643.  This  will  be  readily  perceived  by  Fig.  148,  which  rep- 
resents the  lower  portion  of  the  connecting-rod,  which  works  at 
its  upper  extremity  on  a  pivot  connected  with  the  working  ex- 
tremity of  the  beam. 

The  lower  extremity  of  the  rod 
is  connected  by  a  movable  joint  at 
I,  with  the  lever  I  K.  The  center 
or  axis  to  which  the  rotatory  mo- 
tion is  to  be  communicated,  is  indi- 
cated by  the  letter  K.  Hence  it 
would  appear,  that  as  the  connect- 
ing-rod moves  upward  and  down- 
ward, it  would  carry  the  lever  I  K 
round  the  center  K,  so  as  to  oc- 
cupy successively  the  positions  de- 
noted in  the  figure  by  the  dotted 
shadows  of  the  lever ;  and  thus  a 
continued  rotatory  motion  would  . 
be  communicated  to  the  axis. 

644.  Irregular   Action    of   the 
Crank.  —  On     considering    more 
closely  the  action  of  the  crank,  it 

will  be  found  to  be  by  no  means  continuous  in  its  motion. 
There  are  two  positions  which  the  crank  assumes  in  its  circuit, 
in  which  the  moving  power  has  positively  no  effect  whatever  in 
communicating  a  rotatory  motion  to  it. 

645.  I.    When  the  piston  is  at  the  bottom  of  the  cylinder, 
the  crank  will  be  in  the  position  denoted  in  the  preceding  figure; 
the  joint  I  will  be  in  a  perpendicular  line  between  the  upper 
end  of  the  connecting-rod  and  the  center  K.     It  is  obvious,  that 
as  the  piston  ascends  in  the  cylinder,  the  connecting-rod  will 
tend  to  push  the  joint  I,  not  to  the  right  nor  to  the  left  of  the 
dotted  circle,  but  dir£ctly  downward  upon  the  axis  K. 

646.  II.    When  the  piston  is  at  the  top  of  the  cylinder,  the 
crank  will  have  performed  half  a  revolution,  and  the  joint  I  will 
be  in  a  perpendipular  line  below  the  center  K.     As  the  piston 
descends,  the  connecting-rod  will  tend  to  pull  the  joint  I,  not  to 
the  right  nor  to  the  left  of  the  dotted  circle,  but  directly  up- 


643.  What  are  the  dead  points  in  the  motion  of  the  crank  1    Explain  this  by 
Fig.  148. 


STEAM    ENGINE.  183 

ward  upon  the  axis  K.  It  is  evident,  that  if  in  either  of  these 
positions,  the  action  of  the  crank  were  for  a  moment  to  cease, 
it  would  be  out  of  the  power  of  the  piston  to  put  it  again  into 
motion. 

647.  111.  Another  difficulty  connected  with  the  crank,  is  the 
inequality  of  its  motion.     In  two  positions,  it  has  been  shown 
to  be  actually  stationary.     There  are  also  two  positions,  in  which 
its  action  is  most  energetic ;  and  it  becomes  feebler  in  propor- 
tion as  the  crank  moves  from  these  points  toward  the  two  sta- 
tionary positions  above  described. 

Let  the  reader  once  more  direct  his  attention  to  the  process 
of  drawing  water  from  a  well ;  let  him  imagine  his  own  arm  to 
be  the  connecting-rod ;  and  the  handle  of  the  wheel  the  crank ; 
he  will  find  that  his  force  is  most  effective,  when  the  angle  de- 
scribed by  his  arm  upon  the  crank  is  a  right  angle  ;  and  that 
his  force  will  become  less  effective,  as  the  angle  of  leverage  be- 
comes smaller  or  greater.  The  application  of  this  simple  illus- 
tration to  the  crank  of  the  steam  engine  is  obvious ;  and  the 
result  of  it  is  a  variable,  instead  of  a  uniform,  unremitting  ac- 
tion. In  the  following  paragraph,  a  remedy  for  these  incon- 
veniences will  be  described. 

648.  NATURE  OF  A  FLY-WHEEL. — In  impelling  machinery 
by  force,  it  is  frequently  necessary  that  the  force  should  be  reg- 
ulated.    Jhis  necessity  may  arise  from  several  causes.     There 
may  be  a  want  of  uniformity  in  the  f,rst  moving  power,  as  in 
the  single-acting  engine  of  James  Watt,  in  which  the  descent 
of  the  piston  is  effected  by  the  pressure  of  steam,  while  its  ascent 
is  effected  by  a  totally  different  means.     Or,  there  may  be  a 
want  of  uniformity  in  the  resistance  which  the  force  has  to  over- 
come, as  in  the  crank  described  in  the  preceding  paragraph. 

To  regulate  these  inconveniences  and  equalize  the  motion,  a 
large  heavy  wheel,  called  a  fy-wheel,  is  connected  with  the  ma- 
chinery, so  as  to  receive  its  motion  from  the  impelling  power, 
to  keep  up  the  motion  by  its  own  inertia,  and  distribute  it 
equally  in  all  parts  of  its  revolution.  If  the  moving  power 
slackens,  the  fly-wheel  impels  the  machine  forward  ;  if  the  power 
tends  to  impel  the  machine  too  fast,  the  fly-wheel  slackens  it. 
The  object  of  the  fly-wheel,  therefore,  is  to  absorb,  as  it  were, 
the  surplus  force  at  one  part  of  the  action  of  the  machine,  and 
to  give  it  out  when  the  action  of  the  machine  is  deficient ;  by 
Leslie  it  was  well  compared  to  a  "  reservoir  which  collects  the 
intermittent  currents,  and  sends  forth  a  regular  stream." 

648.  How  does  the  fly-wheel  eontinue  the  motion  of  the  crank  ? 


184  STEAM    ENGINE. 

649.  Connection  of  the  Fly-  Wheel  with  the  Crank. — In  or- 
.der  to  equalize  the  motion  of  the  crank,  Watt  attached  a  fly- 
wheel to  its  axis.     This  wheel  is  constructed  of  large  diameter, 
in  order  that  its  circumference  may  revolve  rapidly :  it  is  of 
great  weight,  being  made  of  iron,  that  it  may  acquire  consider- 
able momentum  so  as  to  render  the  motion  as  uriform  as  pos- 
sible; and  it  is  so  nicely  placed  upon  the  axis,  as  to  be  almost 
free  from  friction,  and  thus  enabled  to  communicate  its  motion 
to  the  axis,  when  this  is  required  from  the  irregular  action  of 
the  crank. 

The  objects  of  the  fly-wheel  in  the  steam  engine,  as  here  de- 
scribed, are  obviously  twofold :  first,  to  extricate  the  machine 
from  the  mechanical  difficulties  which  occur  at  the  two  station- 
ary positions  of  the  crank  ;  and,  secondly,  to  equalize  the  effects 
of  the  varying  leverage  by  which  the  first  mover  acts  on  the 
crank.  But  besides  the  irregularity  in  the  action  of  the  crank, 
there  are  other  causes  which,  in  the  absence  of  a  fly-wheel, 
would  disturb  the  uniform  velocity  of  the  engine  :  there  are 
variations  of  resistance,  and  of  power. 

The  resistance  which  an  engine  has  to  overcome,  particularly 
in  manufactures,  is  continually  liable  to  vary.  When  the  re- 
sistance is  diminished,  the  quantity  of  steam  admitted  through 
the  valves  into  the  cylinder,  is  increased  or  diminished,  as  the 
case  may  be. 

When  the  resistance  is  increased,  or  the  moving  power  dimin- 
ished, the  momentum  accumulated  in  the  fly-wheel  continues 
the  motion  with  little  diminution  of  its  own  velocity.  It  is  not, 
however,  pretended  that  the  equalization  of  force  produced  by 
the  fly-wheel,  is  perfect ;  but  it  is  sufficient  for  ordinary  pur- 
poses ;  and  its  efficiency  will  be  proportioned  to  the  mass  of 
matter  in  the  circumference  of  the  wheel  and  to  the  square 
of  the  wheel's  velocity.  The  next  step  in  the  progress  of  im- 
provement was  to  regulate  the  velocity  of  the  fly-wheel. 

650.  THE  GOVERNOR. — Of  all  the  contrivances  for  regulating 
the  motion  of  machinery,  this  is  said  to  be  the  most  effectual. 
It  will  be  readily  understood  by  the  following  description  of 
Fig.  150.     It  consists  of  two  heavy  iron  balls,  ft,  attached  to 
the  extremities  of  the  two  rods,  b  e.     These  rods  play  on   a 
joint  at  <?,  passing  through  a  mortice  in  the  vertical  stem  d  d. 
At  /,  these  pieces  are  united,  by  joints,  to  the  two  short  rods, 


649.  Why  must  the  fly-wheel  be  of  lar«re  diameter  and  great  weieht  ?  Does  the 
fly-wheel  completely  equalize  the  motion  of  machinery?  650.  What  is  the  gov- 
ernor 1  How  does  the  governor  operate  to  equalize  the  motion  of  machinery  J 


STEAM    ENGINE. 


185 


The  Governor. 


f  A,  -which,  at  their  upper 
ends,  are  again  connected  by 
joints  at  A,  to  a  ring  which 
slides  upon  the  vertical  stem 
d  d.  Now  it  will  be  appa- 
rent that  when  these  balls 
are  thrown  outward,  the 
lower  links  connected  at  /", 
will^  be  made  to  diverge,  in 
consequence  of  which  the  up- 
per links  will  be  drawn  down 
the  ring  with  which  they  are 

connected  at  h.  With  this  ring  at  ?',  is  connected  a  lever  hav- 
ing its  axis  at  g,  and  to- the  other  extremity  of  which,  at  k,  is 
fastened  a  vertical  piece,  which  is  connected  by  a  joint  to  the 
valve  v.  To  the  lower  part  of  the  vertical  spindle  d^s  attached 
a  grooved  wheel  w,  around  which  a  strap  passes,  which  is  con- 
nected with  the  axis  of  the  fly-wheel. 

Now  when  .it  so  happens  that  the  quantity  of  steam  is  too 
great,  the  motion  of  the  fly-wheel  will  give  a  proportionate  ve- 
locity to  the  spindle  d  d,  by  means  of  "the  strap  around  w,  and 
by  which  the  balls,  by  their  centrifugal  force,  will  be  widely 
separated ;  in  consequence  of  which  the  ring  A,  will  be  drawn 
down.  This  will  elevate  the  arm  of  the  lever  &,  and  by  which 
the  end  z,  of  the  short  lever,  connected  with  the  valve  v,  in  the 
steam  pipe,  will  be  raised,  and  thus  the  valve  turned  so  as  to 
diminish  the  quantity  of  steam  admitted  to  the  piston.  When 
the  motion  of  the  engine  is  slow,  a  contrary  effect  will  be  pro- 
duced, and  the  valve  turned  so  that  more  steam  will  be  admit- 
ted to  the  engine. 

651.  Connected  View  of  the  Double-acting  Engine. — We  are 
now  in  a  condition  to  understand  the  relation  which  the  several 
parts  of  the  engine,  already  separately  described,  bear  to  each 
other.  In  Its  general  construction  it  resembles  the  single-acting 
engine  of  Watt  not  described  in  the  present  work,  but  it  differs 
in  several  important  features.  Among  these  are,  its  capability 
of  performing  twice  the  amount  of  work  in  the  same  time,  frcm 
the  simultaneous  action  of  the  pressure,  and  of  the  condensation 
of  steam,  at  each  ascent  and  descent  of  the  piston  ;  its  near  ap- 
proximation to  uniformity  of  power ;  its  economy  of  heat,  and 
consequently  of  fuel,  by  the  diminution  of  cooling  surface;  and 
its  reduced  bulk.  In  the  following  engraving,  taken  from  the 
valuable  work  of  Tredgold,  a  section  of  this  engine  is  illustra- 


186  STEAM    ENGINE. 

ted ;  a  few  additional  remarks  to  those  which  have  already  been 
made  on  its  separate  details,  will  serve  to  explain  its  general 
operation. 

652.  At  the  right  is  seen  (Fig.  151)  the  great  horizontal 
steam  tube  S,  which  admits  steam  into  the  cylinder  through  the 
throttle  valve,  which  appears  near  S  in  the  form  of  a  disc.     The 
boiler  is  omitted  in  the  plate,  but  its  connection  with  the  tube, 
and  the  means  by  which  it  is  supplied  with  warm  water,  may 
be  inferred  from  descriptions  already  given. 

653.  The  double-acting  cylinder  C,  its  two  steam  boxes  and 
four  valves,  and  the  apparatus  for  working  the  valves,. are  the 
next  objects  which  claim  attention.     These  are  explained  by 
Figs.  145  and  146.     The  piston  is  at  the  top  of  the  cylinder. 
The  upper  steam  valve  a  is,  therefore,  represented  as  open  for 
the  admission  of  steam,  the  upper  exhausting  valve  c  as  closed  ; 
the  conditio^i  of  the  two  lower  valves  is  reversed.     The  operation 
of  opening  and  closing  these  four  valves  is  effected  by  a  series 
of  levers,  terminating  in  one  handle  or  spanner,  which  is  worked 
by  two  pegs  attached  to  the  pump-rod  R. 

Before  the  piston  arrives  at  the  bottom  of  the  cylinder,  the 
upper  peg  strikes  the  handle  of  the  levers  downward,  and  in  a 
moment  reverses  the  condition  of  the  four  valves.  The  steam 
from  above  the  piston  th'en  rushes  down  through  the  perpen- 
dicular tube  S,  issues  at  the  lower  steam  valve  c?,  which  will 
now  be  open,  and  forces  up  the  piston ;  but,  before  the  piston 
arrives  at  the  top  of  the  cylinder,  the  lower  peg  strikes  the 
handle  of  the  levers  upward,  the  condition  of  the  valves  is  again 
reversed,  the  steam  below  the  cylinder  rushes  through  the  lower 
exhausting  valve  6  into  the  condenser  B,  and  the  stroke  of  the 
engine  is  repeated. 

654.  In  the  condenser  B,  the  steam  meets  with  a  continual 
jet  of  cold  water.     In  the  double-acting  engine,  condensation 
goes  on  equally  during  the  descent  and  ascent  of  the  piston,  and 
the  condensing  jet  is  therefore  incessantly  at  play;     Engines 
with  a  condenser  are  called  low  pressure  engines.     The  .varia- 
tions which  occur  in  the  velocity  of  the  piston,  and  the  conse- 
quent variations  in  the  quantity  of  steam  dischaFged  -into  the 
condenser,  require  corresponding  variations  in  the  quantity  of 
condensing  water ;  its  amount  is,  therefore,  regulated  by  the  in- 
jection cock,  which  is  worked  by  a  lever  and  handle,  I.     The 
water  produced  by  condensation  of  the  steam  is  removed  by 

652.  In  Fig.  151,  where  is  the  steam  pipe  7    653.  Which  is  the  cylinder  7    654.  Which 
is  the  condenser? 


STEAM   ENGINE. 
FIG.  151.* 


187 


Modern  Steam  Engine. 


188  HIGH    PRESSURE    ENGINE. 

the  air-pump  A,  and  carried  into  the  warm  cistern,  from  which 
a  portion  of  it  is  drawn  by  the  pump  L,  and  conveyed  to  the 
boiler.  The  cistern  containing  the  condenser,  the  air-pump,  and 
the  injection  cock,  is  supplied  with  water  by  the  pump  N,  on 
the  left  side  of  the  beam. 

On  the  extreme  left  is  the  fly-wheel,  a  part  of  which  is  seen 
at  P,  and  to  the  axle  of  which  is  fixed  the  crank,  this  being 
moved  by  the  connecting-rod  attached  to  the  end  of  the  work- 
ing-beam. To  the  fly -wheel  is  also  attached  the  governor,  but 
these  parts  having  already  been  explained,  and  being  unneces- 
sary to  the  understanding  of  the  whole,  are  omitted  in  the 
drawing. 

On  the  right  extremity  of  the  beam  is  seen  the  apparatus 
which  produces  the  parallel  motion.  The  moving  parallelo- 
gram is  represented  at/,  6,  d,  g ;  the  rod  d  c  is  the  radius  rod: 
it  terminates  the  arc  of  the  circle  through  which  the  point  d 
travels.  At  e  is  seen  the  extremity  of  the  pump-rod  R,  which 
is  worked  by  the  same  machinery  as  that  of  the  parallel  motion. 

655.  Returning  to  the  left  side  of  the  beam,  we  find  the 
pumping  apparatus.     D  represents  the  barrel  of  the  pump,  and 
M  is  the  pump-rod,  which  is  connected  with  the  beam  by  me- 
chanism similar  to  that  of  the  parallel  motion,  already  described. 
When  the  piston  of  the  pump  descends,  the  water  is  forced  up- 
ward through  the  pipe  G,  and  conveyed  by  appropriate  chan- 
nels to  a  distance  and  height  proportional  to  the  power  of  the 
engine.     The  barrel  of  the  pump  is  filled  through  the  pipe  F  by 
means  of  machinery  adapted  to  this  purpose  below  ;  and,  when 
the  piston  of  the  pump  ascends,  the  valve  at  the  left  of  the  bar- 
rel opens,  and  the  water  rushes  through  in  the  same  direction 
as  that  from  the  pipe  G.     The  supply  for  the  descent  of  the 
piston  will  rush  in  at  the  bottom  valve  from  F,  and  be  raised 
through  the  pipe  G,  as  before.     The  valves  with  which  the  pis- 
ton of  the  air-pump  is  furnished  are  termed  clacks. 

HIGH   PRESSURE    ENGINE. 

656.  Tn  the  high  pressure  engines,  the  piston  is  pressed  up 
and  down  by  the  force  of  the  steam  alone,  and  without  the 
assistance  of  a  vacuum.     The  additional  power  of  steam  re- 
quired for  this  purpose  is  very  considerable,  being  equal  to  the 
entire  pressure  of  the  atmosphere  on  the  surface  of  the  piston. 
We  have  already  had  occasion  to  show  that  on  a  piston  of  13 

655.  Which  is  the  air-pump  7    Explain  the  water  pump.    656.  What  is  the  differ- 
ence between  the  high  and  low  pressure  engines? 


HORSE    POWER.  189 

inches  in  diameter,  the  pressure  of  the  atmosphere  amounts  to 
nearly  two  tons. 

657.  Now  in  the  low  pressure  engine,  in  which  a  vacuum  is 
formed  on  one  side  of  the  piston,  the  force  of  steam  required  to 
move  it  is  diminished  by  the  amount  of  atmospheric  pressure 
nearly  equal  to  the  size  of  the  piston. 

65*8.  But  in  the  high  pressure  engine,  the  piston  works  in 
both  directions  against  the  weight  of  the  atmosphere,  and  hence 
requires  an  additional  power  of  steam  equal  to  the  weight  of 
the  atmosphere  on  the  piston. 

659.  These  engines  are,  however,  much  more  simple  and  cheap 
than  the  low  pressure,  since  the  condenser,  cold  water  pump, 
air-pump,  and  cold  water  cistern,  are  dispensed  with ;  nothing 
more  being  necessary  than  the  boiler,  cylinder,  piston,  and 
valves.     Hence  for  railroads,  and  all  locomotive  purposes,  the 
high  pressure  engines  are,  and  must  be  used. 

With  respect  to  engines  used  on  board  of  steamboats  the  low 
pressure  are  universally  employed  by  the  English,  and  it  is  well 
known,  that  few  accidents  from  the  bursting  of  machinery  have 
ever  happened  in  that  country.  In  most  of  their  boats  two  en- 
gines are  used,  each  of  which  turns  a  crank,  and  thus  the  neces- 
sity of  a  fly-wheel  is  avoided. 

In  this  country  high  pressure  engines  are  in  common  use  for 
boats,  though  they  are  not  universally  employed.  In  some,  two 
engines  are  worked,  and  the  fly-wheel  dispensed  with,  as  in 
England. 

660.  Accidents. — The  great  number  of  accidents  which  have 
happened  in  this  country,  whether  on  board  of  low  or  high 
pressure  boats,  must  be  attributed  in  a  great  measure,  to  the 
eagerness  of  our  countrymen  to  be  transported  from  place  to 
place  with  the  greatest  possible  speed,  all  thoughts  of  safety 
being  absorbed  in   this   passion.     It  is,   however,  true,   from 
the  very  nature  of  the  case,  that  there  is  far  greater  danger 
from  the  bursting  of  the  machinery  in  the  high,  than  in  the 
low  pressure  engines,  since  not  only  the  cylinder,  but  the  boiler 
and  steam  pipes  must  sustain  a  much  higher  pressure  in  order 
to  gain  the  same  speed,  other  circumstances  being  equal. 

HORSE    POWER. 

661.  When  steam  engines  were  first  introduced,  they  were 
employed  to  work  pumps  for  draining  the  English  coal  mines, 

657.  What  constitutes  a  low  pressure  engine  ?  658.  How  much  more  force  of 
steam  is  required  in  high  than  in  low  pressure  engines  ?  659.  What  parts  are  dis- 
pensed with  in  high  pressure  engines  ? 


190  HCmSE    POWER. 

thus  taking  the  places  of  horses,  which  from  the  earliest  times 
of  using  coal  had  performed  this  service. 

662.  It  being  therefore  already  known  how  many  horses  were 
required  to  raise  a  certain  amount  of  coal  from  a  given  depth, 
the  powers  of  these  engines  were  very  naturally  compared  to 
those  of  horses,  and  thus  an  engine  which  would  perform  the 
work  of  ten  horses,  was  called  an  engine  of  ten  horse  power. 
To  this  day  the  same  term  is  used,  with  the  same  meaning, 
though  very  few  appear  to  know  either  the  origin  of  the  term, 
or  the  amount  of  power  it  implies. 

Several  engineers,  after  the  term  was  thus  used,  made  exper- 
iments, for  the  purpose  of  ascertaining  the  average  strength  of 
horses,  with  a  view  of  fixing  a  standard  of  mechanical  force 
which  should  be  indicated  by  the  term  horse  power. 

This  was  done  by  means  which  it  is  not  necessary  here  to 
describe. 

663.  Smeaton,  a  celebrated  mechanical  philosopher,  estima- 
ted that  the  average  power  of  the  horse,  working  eight  hours  a 
dey,  was  equal  to  the  raising  of  23,000  pounds  at  the  rate  of 
one  foot  per  minute. 

664.  Messrs.   Bolton  and  "Watt  caused  experiments  to  be 
made  with  the  horses  used  in  the  breweries  of  London,  said  to 
be  the  strongest  in  the  world,  and  from  the  result  they  estima- 
ted that  33,000  pounds  raised  at  the  rate  of  one  foot  per  min- 
ute, was  the  value  of  a  horse's  power,  and  this  is  the  estimate 
now  generally  adopted.     When,  therefore,  an  engine  is  said  to 
be  so  many  horses'  power,  it  is  meant  that  it  is  capable  of  over- 
coming a  resistance  equal  to  so  many  times  33,000  pounds 
raised  at  the  rate  of  one  foot  per  minute.     Thus  an  engine  of 
ten  horse  power  is  one  capable  of  raising  a  load  of  330,000 
pounds  one  foot  per  minute,  and  so  at  this  rate,  whether  the 
power  be  more  or  less. 

665.  POWER  OF  STEAM.— Experiment  has  proved   that  an 
ounce  of  water  converted  into  steam  will  raise  a  weight  of  2,160 
pounds  one  foot. 

666.  A  cubic  foot  of  water  contains  1,728  cubic  inches,  and 
the  power,  therefore,  of  a  cubic  foot  of  water,  when  converted 


669.  What  is  said  of  accidents  from  steam  in  our  country  1  661.  Where  did  the  steam 
engine  first  take  the  place  of  horses?  662.  What  is  the  orijrm  of  the  term  horse 
power?  663  What  was  Smeaton's  estimate  of  a  horse's  power  ?  664.  What  was 
Watt  and  Boltou's  estimate  of  a  horse's  power  ?  What  is  meant  by  a  horse's  power 
at  the  present  time?  How  many  horses  would  raise  33.000  poundsone  foot  per  min- 
ute ?  665.  What  is  the  power  of  a  square  inch  of  water  converted  into  steam  ?  666. 
What  is  the  power  of  a  cubic  foot  of  water  converted  into  steam  7  How  much  power 
is  lost  in  acting  upon  the  engine  ? 


HORSE    POWER.  191 

into  steam,  will  be  equal  to  2,160  multiplied  by  1,728,  equal  to 
3,732,480  pounds.  This,  then,  expresses  the  number  of  pounds 
weight  which  a  cubic  foot  of  water  would  raise  one  foot  when 
converted  into  steam,  supposing  that  its  entire  mechanical  force 
could  be  rendered  available.  But  in  practice,  it  is  estimated 
that  the  friction,  and  weight  of  the  machinery  in  action,  require 
about  four-tenths  of  the  whole  force,  while  six-tenths  only  re- 
main as  an  actual  mechanical  power. 

667.  Quantity  of  Water  required  for  each  Horse  Power. — 
One  horse  power,  as  already  explained,  is  equal  to  a  force  which 
will  raise  33,000  pounds  one  foot  high  per  minute.     This  being 
multiplied  by  60  will  show  the  force  required  to  raise  the  same 
weight  at  the  rate  of  one  foot  per  hour,  namely,  33,000  X  60  = 
1,980,000  pounds. 

668.  Now  the  quantity  of  water  required  for  this  effect,  will 
be  found  by  considering,  as  already  shown,  that  a  cubic  inch  of 
water  in  the  form  of  steam,  is  equal  to  a  force  raising  2,160 
pounds  a  foot.     If  we  divide  1,980,000,  therefore,  by  2,160,  we 
shall  have  the  number  of  cubic  inches  of  water  required  to  pro- 
duce a  one  horse  power,  namely,  9.160.     But  we  have  already 
shown  that  only  6  parts  out  of  1 0  of  the  force  of  steam  can  be 
calculated  on  as  a  moving  power,  4  parts  being  expended  on 
the  action  of  the  enjrine.     To  find,  then,  the  amount  of  wa>te 
in  916  cubic  inches  of  water,  we  must  divide  that  number  by  6, 
and  multiply  the  result  by  4,  when  we  shall  have  610  as  the 
number  of  cubic  inches  of  water  wasted.     The  total  quantity  of 
water,  therefore,  which  is  turned  into  steam  per  hour,  to  pro- 
duce a  one  horse  power,  is  equal  to  610  added  to  916,  namely, 
1,526  cubic  inches.     Hence  we  see  the  necessity  of  the  immense 
capacities  of  the  boilers  of  large  steamboats. 

669.  Amount  of  Mechanical  Virtue  in  Coal. — For  more  than 
thirty  years,  the  engineers  of  many  of  the  English  coal  mines 
have  published  annual  accounts  of  their  experiments  with  the 
steam  engines  under  their  care,  for  the  purpose  of  ascertaining 
the  exact  amount  of  coal  required  to  perform  certain  duties. 
The  results  of  these  experiments  are  among  the  most  curious  and 
instructive  facts  which  the  lights  of  science  at  the  present  day, 
have  thrown  upon  the  manufacturing  arts.     They  were  entirely 
unexpected  to  the  owners  of  the  mines,  and  equally  so  to  men 
of  science. 

667.  How  many  cubic  inches  of  water  is  required  to  produce  a  one  horse  power? 
668.  How  do  you  find  how  many  cubic  inches  of  water  there  are  in  a  one  horse 
power  1  669.  What  amount  of  weight  is  it  said  a  bushel  of  coal  will  raise  by  means 
of  steam!  What  was  the  weight  raised  by  (he  second  trial  1 


192  LOCOMOTIVE. 

In  the  report  of  the  engineers  thus  employed,  for  1835,  it 
was  announced  that  a  steam  engine  employed  at  a  copper  mine 
in  Cornwall,  had  raised,  as  its  average  work,  95  millions  of 
pounds  a  foot  high,  with  a  single  bushel  of  bituminous  coal. 

This  mechanical  effect  was  so  enormous  and  so  unexpected, 
that  the  best  judges  of  the  subject  considered  it  beyond  the 
bounds  of  credulity ;  the  proprietors,  therefore,  agreed  that  an- 
other trial  should  be  made  in  the  presence  of  competent  wit- 
nesses :  when,  to  the  astonishment  of  all,  the  result  exceeded 
the  former  report  by  30  millions  of  pounds.  In  this  experi- 
ment, for  every  bushel  of  coal  consumed  under  the  boiler,  the 
engine  raised  125-i-  millions  of  pounds  one  foot  high. 

670.  On  this  subject,  Dr.  Lardner,  in  his  treatise  on  the  steam 
engine,  has  made  the  following  calculations : — 

A  bushel  of  coal  weighs  84  pounds,  and  can  lift  56,027  tons 
a  foot  high,  therefore,  a  pound  of  coal  would  raise  667  tons  to 
the  same  height ;  and  an  ounce  would  raise  42  tons  one  foot 
high,  or  it  would  lift  18  pounds  a  mile  high. 

Since  a  force  .of  18  pounds  is  capable  of  drawing  two  tons 
upon  a  railway,  it  follows  that  an  ounce  of  coal  would  draw  2 
tons  a  mile,  or  1  ton  two  miles.  (In  the  common  engines,  how- 
ever, the  actual  consumption  of  coal  is  equal  to  about  8  ounces 
per  ton  for  every  mile.) 

The  great  Egyptian  pyramid  has  a  base  of  700  feet  each  way, 
and  is  500  feet  high ;  its  weight  amounting  to  12,760,000,000 
pounds.  To  construct  it,  is  said  to  have  cost  the  labor  of  100,- 
000  men  for  20  years.  Yet  according  to  the  above  calculations, 
its  materials  could  have  been  raised  from  the  ground  to  their 
present  positions  by  the  combustion  of  479  tons  of  coal. 

LOCOMOTIVE. 

671.  This  word,  from  the  Latin,  means  "moving  from  place 
to  place,"  and  is  applied  to  steam  engines  used  on  railroads. 

Our  limits  will  only  allow  a  short  description  of  this  wonder- 
working machine,  which,  during  the  last  quarter  of  a  century, 
has  been  the  means,  'with  respect  to  locomotion,  of  converting 
days  into  hours,  and  weeks  into  days. 

The  principal  external  parts  of  a  locomotive  are  indicated  by 
the  letters  on  Fig.  152. 

670.  What  weight  will  a  pound  of  coal  raise  ?  How  great  a  force  mav  an  ounce  of 
coal  be  made  to  produce!  What  is  the  size  and  weight  of  the  great  pyramid  of 
Egypt  1  What  weight  of  coal  would  be  required  to  raise  its  materials  to  their  present 
elevation  ? 


LOCOMOTIVE. 


193 


672.  The  truck  wheels,  A  A,  are  of  cast  iron,  about  two  and 
a  half  feet  in  diameter,  all  Of  them  connected  by  an  iron  frame, 
in  the  center  of  which,  the  end  of  the  boiler  rests  on  a  pivot,  so 
as  to  allow  a  revolving  motion,  in  order  to  accommodate  the 
engine  to  short  curves  in  the  road. 


FIG.  152. 


673.  The  boiler  B,  which  makes  the  chief  bulk  of  the  loco- 
motive, is  of  rolled  iron,  about  12  feet  long,  of  great  weight,  and 
strength,  to  resist  the  pressure  of  the  steam.  It  is  put  together 
by  iron  bolt*,  only  an  inch  or  two  apart,  so  as  to  be  perfectly 
steam-tight  under  the  greatest  force. 


672.  What  are  the  truck  wheels  of  a  locomotive,  and  why  do  they  revolve  on  a  cen- 
fw?    673.  What  forms  the  chief  bulk  of  the  locomotive  1 


194  LOCOMOTIVE. 

Above  R  is  the  fire-box,  with  a  door,  not  shown,  for  admis- 
sion of  the  wood.  The  interior  of  the  boiler  is  composed  of 
about  100  copper  tubes,  through  which  the  smoke  and  heat 
pass  to  the  chimney.  These  tubes  are  entirely  surrounded  by 
water,  which  the  heat  emitted  by  the  tubes,  as  they  pass  through 
it,  converts  into  steam. 

674.  The  pump  P,  supplies  the  boiler  with  water,  which  it 
takes  from  the  tender,  not  shown,  but  which  is  connected  with 
the   locomotive,  and   on  which  the  fuel  is  carried.     In  cold 
weather,  the  waste  steam  heats  this  water  before  it  is  admitted 
to  the  toiler. 

675.  The  steam  cylinder  0,  communicates  with  the  boiler  by 
a  short  pipe,  for  admission  of  the  steam.     In  this  cylinder  works 
the  piston,  which  gives  motion  to  the  engine. 

The  cylinder  is  externally  of  brass,  kept  polished  in  order  to 
prevent  the  radiation  of  the  heat.  Its  diameter  is  about  12 
inches,  and  the  movement  of  the  piston  20  inches.  This  is 
furnished  with  valves,  working  in  the  same  manner  as  those 
already  described  for  the  steam  engine. 

The  alternate,  horizontal  motion  of  the  piston,  is  so  connected 
with  the  driving  wheels,  as  to  give  them  a  rotatory  motion,  by 
which  the  engine  is  moved. 

676.  This  is  done  by  means  of  the  connecting-rods  R  R,  which 
are  jointed  to  the  spokes  of  the  drivers  at  one  end,  and  to  the 
piston  rod  I,  at  the  other,  thus  connecting  the  force  of  the  steam 
with  that  part  of  the  engine  by  which  the  whole  is  actuated. 

The  immense  force  which  the  steam  exerts,  is  shown  by  the 
power  required  to  draw  20  or  30  cars,  loaded  with  hundreds 
of  tons,  at  the  rate  of  20  or  30  miles  an  hour.  And  yet  a 
single  locomotive'will  draw  such  a  load  even  up  an  inclined 
plane. 

677.  The  driving  wheels  D  D,  by  which  the  locomotive  is 
moved,  are  of  cast  iron,  with  strong  wrought  iron  tire,  so  as  to 
withstand  any  shock  which  it  is  considered  possible  to  happen, 
since  on  the  strength  of  these,  the  lives  of  hundreds  of  passen- 
gers may  depend,  as  the  fracture  of  one  of  them  may  cast  the 
engine  and  entire  train  from  the  rails.     In  diameter,  they  are 
from  5  to  6  feet. 

678.  These  four  wheels  are  connected  together,  not  only  by 
the  connecting-rods,  but  also  by  a  strong  iron  frame,  and  by  the 

675.  Describe  the  steam  cylinder,  and  tell  its  use.  676.  What  are  the  connecting- 
rods?  677.  What  are  the  wheels  which  give  motion  to  the  locomotive  1  Why  are 
the  driving  wheels  made  of  great  strength  ?  678.  Why  are  these  fotir  wheels  con- 
nected 7  On  what  principle  do  these  wheels  apt  1 


LOCOMOTIVE.     N  195 

axle-trees  which  revolve  with  them,  so  that  the  greatest  amount 
of  adhesion  to  the  rails  is  obtained.  This  is  a  most  important 
point  in  the  construction  of  the  engine,  since  by  this  means  all 
the  wheels  must  act  together,  thus  forming  by  their  adhesion 
to  the  rails  a  single  fulcrum,  acting  as  a  lever  of  the  third  kind, 
(322,)  of  which  the  spoke  of  the  wheel  is  the  lever,  and  the  pis- 
ton, through  the  connecting-rods,  the  power.  It^  therefore,  one 
of  the  wheels  slips  on  the  rail,  they  must  all  slip,  it  being  this 
connection  by  which  locomotives  draw  such  enormous  loads 
over  inclined  planes. 

679.  The  lever  L,  opens  the  throttle  valve,  by  which  the 
steam  is  admitted  to  the  cylinder,  from  the  boiler.     When  the 
engine  is  to  be  started,  the  engineer  opens  this  communication ; 
when  the  piston  begins  its  alternate  motion ;  the  drivers  their 
revolutions,  and  the  engine  and  train  their  progress. 

680.  The  reversing  handle  H,  acts  on  machinery  for  that 
purpose,  in  such  a  manner  as  to  reverse  the  motion  of  the  driv- 
ing wheels,  giving  them  a  backward  instead  of  a  forward  action, 
in  a  moment.     It  is  used  whenever  there  is  danger  of  a  colli- 
sion, or  when  it  is  desired  to  give  the  engine  a  reverse  move- 
ment on  any  occasion. 

The  spring  balance  N,  is  connected  with  a  graduated  scale  by 
which  the  pressure  of  the  steam  is  indicated. 

681.  The  safety  valve  lever  S,  is  connected  with  a  valve,  so 
constructed  .as  to  open  when  the  pressure  is  above  a  certain 
amount,  and  thus  allow  the  steam  to  escape.     When  properly 
adjusted,  this  may  be  the  means  of  saving  the  engine  from  one 
of  the  most  fearful  of  accidents,  that  of  bursting  the  boiler. 

682.  The  smoke  pipe  M,  is  connected  with  the  fire-box,  by 
means  of  the  copper  tubes  running  through  the  boiler,  already 
mentioned.     Various  contrivances  have  been  invented  to  arrest 
the  sparks  which  are  emitted  with  the  smoke,  and  which  have 
often  set  fire  to  bridges  and  other  buildings.     For  this  purpose 
a  wire  gauze  placed  across  the  mouth  of  the  pipe,  has  been  the 
most  efficient. 

The  engine  frame  F,  is  made  of  wrought  iron,  strongly  con- 
nected by  rivets,  and  to  which  all  parts  of  the  locomotive  are 
attciched,  and  by  which  they  are  combined  into  a  single  instru- 
ment, to  be  moved  forward  as  a  great  power,  by  means  of  which 
hundreds  of  tons  are  to  follow. 


679.  Describe  the  manner  of  starting  the  engine.  680.  What  is  the  use  of  the  re- 
Tersing  handle.  681.  How  does  the  safety  valve  act,  and  for  what  purpose  ?  682. 
What  are  the  means  of  arresting  sparks  from  the  smoke  pipe  ? 


.   ••- 
196  THE    RAILS. 

The  valve  box  V,  contains  the  valves  of  the  cylinder,  which 
have  already  been  described  while  treating  of  the  steam  engine. 

The  steam  whistle  U,  is  composed  of  a  cylinder,  with  peculiar 
internal  arrangements,  on  which  the  steam  from  the  boiler  be- 
ing admitted,  by  a  valve,  makes  a  well-known  sound,  heard  at 
the  distance  of  many  miles.  By  its  report,  the  degrees  of  press- 
ure of  the  steam  are  indicated. 

The  slide  valve  rod  K,  works  the  valve  by  which  the  steam 
is  admitted  from  the  boiler  to  the  cylinder,  and  by  which  the 
piston  is  moved. 

683.  Springs  of  the  Boiler. — The  boiler  rests  on  steel  springs, 
composed  of  many  flat  pieces  of  different  lengths,  laid  one  on 
the  other,  forming  a  pyramidal  pile  six  or  eight  inches  high, 
and'  of  sufficient  strength  to  bear  many  tons.  By  the  slight 
motion  of  these  springs,  the  concussion  between  the  engine  and 
the  rails  is  prevented,  and  without  which  neither  would  preserve 
its  integrity  for  an  hour,  under  the  tremendous  shocks,  the 
weight  and  motion  of  the  engine  sometimes  give. 


ADHESION    TO    THE    RAILS. 


684.  We  have  already  noticed  the  necessity  of  so  combining 
the  action  of  the  driving  wheels,  as  to  make  them  form  an  in- 
dividual fulcrum,  by  their  adhesion  to  the  rails. 

On  this  the  motion  of  the  engine,  and  consequently  of  the 
whole  train  depends,  and  hence  the  necessity  of  the  enormous 
weight  of  the  locomotive.  On  roads,  through  hilly  sections  of 
the  country,  the  weight  of  the  engine  is  made  to  correspond  to 
the  inclination  of  the  grade.  Were  this  not  the  case,  as  the 
adhesion  depends  on  the  weight,  the  wheels  would  revolve  with- 
out advancing,  and  thus  the  whole  train  would  remain  motion- 
less, because  the  weight,  with  the  inclination,  required  a  greater 
force  than  the  power  of  the  engine.  On  such  roads  where 
heavy  freight  trains  are  to  be  drawn,  the  weight  of  the  engine 
sometimes  amounts  to  40  or  50,000  pounds. 

685.  In  all  cases,  the  invariable  condition  must  be,  that  the 
force  be  greater  than  the  resistance,  otherwise  no  progress  will 
be  effected ;  and  as  we  have  already  seen,  the  force  depends  on 
the  adhesion,  and  this  on  the  weight,  so  it  is  obvious  that  a 

683.  What  is  said  of  the  springs  on  which  the  boiler  rests?  684.  Why  are  the 
driving  wheels  so  connected  as  to  form  an  individual  fulcrum  ?  What  is  the  weight 
of  some  engines'?  685.  What  is  said  of  the  proportion  between  the  weight  of  the  en- 
gine and  the  grade  of  the  road?  What  must  be  the  condition  with  respect  to  the 
weight  and  force  ?  What  is  the  estimate  between  the  force  of  adhesion  and  -the 
weight  of  the  drivers! 


THE    RAILS. 


197 


FIG.  153. 


ponderous  engine  only,  will  draw  a  heavy  train  over  a  rapid 
inclination.  It  is  estimated  that  the  force  of  adhesion  amounts 
to  one-sixth  of  the  weight  of  the  drivers  on  the  rails. 

686.  Section  of  the  Boiler. — It  has  been  noticed  above,  that 
locomotive  boilers  are  furnished  with  copper  tubes,  passing  from 
the  fire-box  to  the  chimney.     The  ordinary  number  of  these 
tubes  is  120,  and  their  diameters  about  two  inches.     If  larger 
than  this,  they  are  liable  to  collapse  by  the  pressure  of  the 
steam,  and  if  smaller,  they  soon  become  clogged  by  the  soot. 

The  end  of  such  a  boiler  is  repre- 
sented by  Fig.  153.  The  fire-box 
with  the  grate  for  fuel,  is  seen  at  B, 
above  which  are  the  ends  of  the  tubes. 
In  Fig.  152,  these  parts  are  above  R. 
A  shows  the  dome,  above  the  fire-box, 
and  which  forms  a  part  of  the  boiler, 
being  open  and  containing  the  steam 
as  it  is  formed. 

687.  The  steam  is  conveyed  to  the 
cylinders  from  the  large  pipe,  seen  at 
the  upper  part  of  the  dome,  the  two 
arrows  showing  that  it  is  admitted 
from  all  directions.     The  mouth  of 
this  pipe  is  thus  elevated,  in  order  to 
avoid  the  admission  of  the  water  when 
in  the  state  of  the  greatest  ebullition. 

The  fire-box  is  made  of  thick,  rolled 
iron,  with  double  walls,  about  three 

inches  apart,  the  space  between  them  being  filled  with  water, 
so  that  the  fire  is  surrounded  with  water,  except  at  the  door 
where  the  fuel  is  admitted. 

688.  The  water  is  pumped  into  the  side  of  the  fire-box  at  C, 
whTch  opens  into  the  boiler. 

The  boiler  is  only  about  half  filled  with  water,  the  upper  part 
being  devoted  to  steam. 

The  boiler  is  made  of  thiok,  rolled  iron,  strongly  riveted  to- 
gether, and  in  the  form  of  a  cylinder,  being  that  which  best 
resists  the  pressure  of  the  steam. 

In  order  to  confine  the  heat,  or  prevent  radiation,  boilers  are 
covered  with  wood,  in  the  form  of  narrow  strips  of  board,  over 
which  there  is  a  covering  of  sheet  iron. 

686.  Show  by  Fig.  153,  the  situations  of  the  fire-box,  steam  pipe,  tubes  and  grate. 
687.  Why  is  the  mouth  of  the  steam  pipe  so  high  in  the  dome  7  688.  Where  is  the 
water  admitted  to  the  boiler. 


Fire-Box  and  Boiler. 


198 


ACOUSTICS. 


689.  Locomotive  engines  are  always  on  the  high  pressure 
principle,  because  such  engines  are  more  simple  in  structure 
than  those  of  low  pressure,  the  former  not  requiring  the  con- 
densing apparatus  which  is  indispensable  in  the  latter. 


CHAPTER  X. 

ACOUSTICS. 

690.  Acoustics  is  that  branch  of  natural  philosophy  which 
treats  of  the  origin,  propagation,  and  effects  of  sound. 

691.  Vibration  of  Solids. — When  a  sonorous,  or  sounding 
body  is  struck,  it  is  thrown  into  a  tremulous  or  vibrating  mo- 
tion.    This  motion  is  communicated  to  the  air  which  surrounds 
us,  and  by  the  air  is  conveyed  to  our  ear  drums,  which  also 
undergo  a  vibratory  motion,  and  this  last  motion  throwing  the 
auditory  nerves  into  action,  we  thereby  gain  the  sensation  of 
sound. 

If  any  sounding  body,  of  considerable  size,  is  suspended  in 
the  air  and  struck,  this  tremulous  motion  is  distinctly  visible 
to  the  eye,  and  while  the  eye  perceives  its  motion,  the  ear  per- 
ceives the  sound. 

692.  Proof  by  the  Air-Pump, — That 
sound  is  conveyed  to  the  ear  by  the  motion 
which  the  sounding  body  communicates  to 
the  air,  is  proved  by  an  interesting  experi- 
ment with  the  air-pump. 

693.  This  is  done  by  a  little  piece  of  me- 
chanism shown  by  Fig.  154.     It  consists 
of  a  block  of  lead  weighing  a  pound  or  two, 
into  which  is  inserted  the  standard  of  the 
bell  A.     A  piece  of  wire,  also  fixed  to  the 
lead,  is  bent  over  the  bell  at  B,  to  which  is 
jointed  the  handle  of  a  small  hammer.     At 
half  an  inch  from   the  joint,  the  handle 
passes  through  the  end  of  the  sliding  rod 

689.  Why  are  locomotives  on  the  high  pressure  principle  ?  690.  What  is  acoustics  7 
691.  When  a  sonorous  body  is  struck  within  hearing,  in  what  manner  do  we  gain 
from  it  the  sensation  of  sound  1  692.  How  is  it  proved  that  sound  is  conveyed  to  the 
ear  by  the  medium  of  the  air  7  693.  Describe  the  mechanism,  Fig.  154,  by  which 
this  is  proved. 


FIG.  154. 


About  Sound. 


DIVING    BELL.  199 

C,  which  passes  air-tight  through  the  stuffed  collar  of  the  glass 
receiver  D. 

Now  it  is  obvious  by  the  figure,  that  on  working  the  sliding- 
rod  by  its  handle,  the  hammer  will  strike  the  bell,  the  sound  of 
which  may  be  heard  to  a  considerable  distance.  But  if  the  re- 
.ceiver  be  set  on  the  plate  of  an  air-pump,  and  the  air  exhausted, 
its  sound  will  become  less  and  less  audible,  until  a  vacuum  is 
formed,  when,  although  the  hammer  is  made  to  strike  the  bell, 
no  sound  will  be  heard.  The  lead  should  be  placed  on  a  piece 
of  cotton  batting,  so  as  not  to  transmit  the  sound  through  the 
solid  on  which  it  stands. 


DIVING   BELL. 

694.  On  the  contrary,  when  the  air  is  more  dense  than  or- 
dinary, or  when  a  greater  quantity  is  contained  in  a  vessel,  than 
in  the  same  space  in  the  open  air,  the  effect  of  sound  on  the 
ear  is  increased.     This  is  illustrated  by  the  use  of  the  diving 
bell 

The  diving  bell  is  a  large  vessel,  open  at  the  bottom,  under 
which  men  descend  to  the  beds  of  rivers,  for  the  purpose  of  ob- 
taining articles  from  the  wrecks  of  vessels.  When  this  machine 
is  sunk  to  any  considerable  depth,  the  water  above,  by  its  press- 
ure, condenses  the  air  under  it  with  greafr  force*  In  this  situa- 
tion, a  'whisper  is  as  loud  as  a  common  voice  in  the  open  air, 
and  an  ordinary  voice  becomes  painful  to  the  ear. 

695.  Effects  in  high  Places. — Again,  on  the  tops  of  high 
mountains  where  the  pressure,  or  density  of  the  air  is  much  less 
than  on  the  surface  of  the  earth,  the  report  of  a  pistol  is  Eeard 
only  a  few  rods,  and  the  human  voice  is  so  weak  as  to  be  in- 
audible at  ordinary  distances. 

Thus,  the  atmosphere  which  surrounds  us,  is  the  medium  by 
which  sounds  are  conveyed  to  our  ears,  and  to  its  vibrations  we 
are  indebted  for  the  sense  of  hearing,  as  well  as  for  all  we  enjoy 
from  the  charms  of  music. 

696.  Solids  conduct  Sound. — The  atmosphere, -though  the 
most  common,  is  not,  however,  the  only,  or  the  best  conductor 
of  sound.     Solid  bodies  conduct  sound  better  than  elastic  riuids. 
Hence,  if  a  person  lay  his  ear  on  a  long  stick  of  timber,  the 
scratch  of  a  pin  may  be  heard  from  the  other  end,  which  could 
not  be  perceived  through  the  air. 

694.  When  the  air  is  more  dense  than  ordinary,  how  does  it  affect  sound?  695. 
What  is  said  of  the  effects  of  sound  on  the  tops  of  high  mountains  1  696.  Which  ar« 
the  best  conductors  of  sound,  solid  or  elastic  substances  1 


200  ACOUSTICS. 

697.  The  earth  conducts  loud  rumbling  sounds  made  below 
its  surface  to  great  distances.     Thus,  it  is  said,  that  in  countries 
where  volcanoes  exist,  the  rumbling  noise  which  generally  pre- 
cedes an  eruption,  is  heard  first  by  the  beasts  of  the  field,  be- 
cause their  ears  are  commonly  near  the  ground,  and  that  by 
their  agitation  and  alarm,  they  give  warning  of  its  approach  to- 
the  inhabitants.  , 

698.  The  Indians  of  our  country,  by  laying  their  ears  on  the 
ground,  will  discover  the  approach  of  horses  or  men  when  they 
are  at  such  distances  as  not  to  be  heard  in  any  other  manner. 

699.  Velocity  of  Sound. — Sound  is  propagated  through  the 
air  at  the  rate  of  1,142  feet  in  a  second  of  time.     When  com- 
pared with  the  velocity  of  light,  it  therefore  moves  but  slowly. 
Any  one  may  be  convinced  of  this  by  watching  the  discharge 
of  cannon  at  a  distance.     The  flash  is  seen  apparently  at  the 
instant  the  gi-nner  touches  fire  to  the  powder;  the  whizzing  of 
the  ball,  if  the  ear  is  in  its  direction,  is  next  heard,  and  lastly, 
the  report. 

700.  Blot's  Experiment. — Solid   substances  convey  sounds 
with  greater  velocity  than  air,  as  is  proved  by  the  following  ex- 
periment, made  at  Paris,  by  M.  Biot:  — 

At  the  extremity  of  a  cylindrical  tube,  upward  of  3,000  feet 
long,  a  ring  of  metal,  was  placed,  of  the  same  diameter  as  the 
aperture  of  the  tube ;  and  in  the  center  of  this  ring,  in  the 
mouth  of  the  tube,  was  suspended  a  clock-bell  and  hammer. 
The  hammer  was  made  to  strike  the  ring  and  the  bell  at  the 
same  instant,  so  that  the  sound  of  the  ring  would  be  transmit- 
ted to  the  remote  end  of  the  tube,  through  the  conducting 
power  of  the  tube  itself,  while  the  sound  of  the  bell  would  be 
transmitted  through  the  medium  of  the  air  inclosed  in  the  tube. 
The  ear  being  then  placed  at  the  remote  end  of  the  tube,  the 
sound  of  the  ring,  transmitted  by  the  metal  of  the  tube,  was 
first  heard  distinctly,  and  after  a  short  interval  had  elapsed,  the 
sound  of  the  bell  transmitted  by  the  air  in  the  tube,  was  heard. 
The  result  of  several  experiments  was,  that  the  metal  conducted 
the  sound  at  the  rate  of  about  11,865  feet  per  second,  which  is 
about  ten  and  a  half  times  the  velocity  with  which  it  is  con- 
ducted by  the  air. 

701.  Sound  moves  forward  in  straight  lines,  and  in  this  re- 


697.  What  is  said  of  the  earth  as  a  conductor  of  sounds'?  608.  Ho\v  is  it  said  that 
the  Indians  discover  the  approach  of  horses  1  699.  How  fast  does  sound  pass  through 
t'he  air  ]  What  is  said  of  the  firing  of  cannon  with  respect  to  sound  7  700.  Which 
convey  sounds  with  the  greatest  velocity,  solid  substances,  or  air  1 


ACOUSTICS. 


201 


spect  follows  the  same  laws  as  moving  bodies,  and  light.  It 
also  follows  the  same  laws  in  being  reflected,  or  thrown  back, 
when  it  strikes  a  solid,  or  reflecting  surface. 

7U2.  ECHO. — If  the  surface  be  smooth,  and  oft  considerable 
dimensions,  the  sound  will  be  reflected,  and  an  echo  will  be 
heard ;  but  if  the  surface  is  very  irregular,  soft,  or  small,  no 
such  effect  will  be  produced. 

]n  order  to  hear  the  echo,  the  ear  must  be  placed  in  a  certain 
direction,  in  respect  to  the  point  where  the  sound  is  produced, 
and  the  reflecting  surface. 

If  a  sound  be  produced  at  A,  Fig.  155,  and  strike  the  plane 
surface  B,  it  will  be  reflected  back  in  the  same  line,  and  the 
echo  will  be  heard  at  C  or  A.  That  is,  the  angle  under  which 
it  approaches  the  reflecting  surface,  and  that  under  which  it 
leaves  it,  will  be  equal. 

FIG.  157. 


A. 

Echo. 


Reverberation. 


FIG.  156. 


Reflection  of  Sound. 


703.  Whether  the  sound  strikes  the  reflecting  surface  at  right 
angles,  or  obliquely,  the  angle  of  approach,  and  the  angle  of  re- 
flection, will  always  be  the  same,  and  equal. 


700.  Describe  the  experiment,  proving  that  sound  is  conducted  by  a  metal  with 
greater  velocity  than  by  the  air.  701.  In  what  lines  does  sound  move?  703.  Explain 
Fijr.  156,  and  show  in  what  direction  sound  approaches  and  leaves  a  reflecting 
surface.  r>* 


202 


ACOUSTICS. 


Tliis  is  illustrated  by  Fig.  156,  where  suppose  a  pistol  to  be 
fired  at  A,  while  the  reflecting  surface  is  at  C  ;  then  the  echo  will 
be  heard  at  B,  the  angles  2  and  1  being  equal  io  each  other. 

704.  Reverberation  of  Sound. — If  a  sound  be  emitted  be- 
tween two  reflecting  surfaces,  parallel  to  each  other,  it  will  rever- 
berate, or  be  answered  backward  and  forward  several  times. 

Thus,  if  the  sound  be  made  at  A,  Fig.  157,  it  will  not  only 
rebound  back  again  to  A,  but  will  also  be  reflected  from  the 
points  C  and  D,  and  were  such  reflecting  surfaces  placed  at 
every  point  around  a  circle  from  A,  the  sound  would  be  thrown 
back  from  them  all,  at  the  same  instant,  and  would  meet  again 
at  the  point  A. 

We  shall  see,  under  the  article  Optics,  that  light  observes 
exactly  the  same  law  in  respect  to  its  reflection  from  plane  sur- 
faces, and  that  the  angle  at  which  it  strikes,  is  called  the  angle 
of  incidence,  and  that  under  which  it  leaves  the  reflecting  sur- 
face, is  called  the  angle  of  reflection.  The  same  terms  are  em- 
ployeM  in  respect  to  sound. 

705.  Reflection  in  a  Circle. — In  a  circle,  sound  is  reflected 
from  every  plane  surface  placed  around  it,  and  hence,  if  the 
sound  is  emitted  from  the  center  of  a  circle,  this  center  will  be 
the  point  at  which  the  echo  will  be  most  distinct. 

Suppose  the  ear  to  be  placed  at 
the  point  A,  Fig.  158,  in  the  cen- 
ter of  a  circle ;  and  let  a  sound  be 
produced  at  the  same  point,  then 
it  will  move  along  the  line  A  E, 
and  be  reflected  from  the  plane  sur- 
face, back  on  the  same  line  to  A ; 
and  this  will  take  place  from  all  the 
plane  surfaces  placed  around  the 
circumference  of  a  circle ;  and  as 
all  these  surfaces  are  at  the  same 
distance  from  the  center,  so  the  re- 
flected sound  will  arrive  at  the  point 
A,  at  the  same  instant;  and  the 
echo  will  be  loud,  in  proportion  to  the  number  and  perfection 
of  these  reflecting  surfaces. 

706.  WHISPERING  GALLERY. — It  is  apparent  that  the  audi- 
tor, in  this  case,  must  be  placed  in  the  center  from  which  the 

704.  What  is  the  angle  under  which  sound  strikes  a  reflecting  surface  called? 
What  is  the  angle  under  which  it  leaves  a  reflecting  surface  called?  Is  there  any 
difference  in  the  quantity  of  these  two  angles?  705.  Suppose  a  pistol  to  be  fired  in 
the  center  of  a  circular  room,  where  would  be  the  echo  ? 


FIG  158. 


Reflection  in  a  Circle. 


ACOUSTICS.  203 

s 

sound  proceeds,  to  receive  the  greatest  effect.  But  if  the  shape 
of  th«  room  be  oval,  or  elliptical,  the  sound  may  be  made  in 
one  part,  and  the  echo  will  be  heard  in  another  part,  because 
the  ellipse  has  two  points,  called  foci,  at  ^ne  of  which,  the  sound 
being  produced,  it  will  be  concentrated  in  the  other. 

Suppose  a  sound  to  be  produced  at 
A,  Fig.  159,  it  will  be  reflected  from  FIG.  159. 

the  sides  of  the  room,  the  angles  of  in- 
cidence being  equal  to  those  of  reflection, 
and  will  be  concentrated  at  B.  Hence, 
a  hearer  standing  at  B,  will  be  affected 
by  the  united  rays  of  sound  from  differ- 
ent parts  of  the  room,  so  that  a  whisper 
at  A,  will  become  audible  at  B,  when  it 
would  not  be  heard  in  any  other  part  of 
the  room.  Were  the  sides  of  the  room 
lined  with  a  polished  metal,  the  rays  of 
light  or  heat  would  be  concentrated  in 

the  same  manner.  Whispering  Gallery. 

The  reason  of  this  will  be  understood, 

when  we  consider  that  an  ear,  placed  at  C,  will  receive  only  one 
ray  of  the  sound  proceeding  from  A,  while  if  placed  at  B,  it 
will  receive  the  rays  from  all  parts  of  the  room.  Such  a  room, 
whether  constructed  by  design  or  accident,  would  be  a  whisper- 
ing gallery. 

7  07.  Successive  Reflections  of  Sound. — "Several  reflecting 
surfaces  may  be  so  situated  in  respect  to  distance  and  direction, 
that  a  sound  proceeding  from  a  certain  point,  will  be  reflected, 
first  from  one  surface,  and  then  from  another,  at  a  little  dis- 
tance, afterward  from  a  third,  and  so  on  ;  or  it  may  be  reflected 
from  the  first  surface  to  the  second,  and  from  the  second  to  the 
third,  and  from  this  to  a  fourth,  and  so  on,  even  it  is  said,  to 
the  number  of  eight  or  ten." 

708.  According  to  the  distance  at  which  the  speaker  stands, 
a  reflecting  surface  will  return  the  echo  of  several,  or  of  fewer 
syllables ;  for  in  order  to  avoid  confusion,  all  the  syllables  must 
be  uttered  before  the  echo  of  the  first  syllable  reaches  the  ear. 
In  a  moderate  way  of  speaking,  about  3^-  syllables  are  pro- 
nounced in  one  second,  or  seven  syllables  in  two  seconds. 

706.  Explain  Fig.  159,  and  give  the  reason.-  Suppose  a  sound  to  be  produced  in 
one. of  the  foci  of  an  ellipse,  where  then  might  it  be  most  distinctly  heard  ?  707. 
What  number  of  echoes  are  said  to  happen  from  one  sound  ?  708-  How  many  sylla- 
bles are  pronounced  in  a  second  1  When  an  echo  repeats  seven  syllables,  how  far 
off  is  the  reflecting  surface  ?  Explain  this. 


204  ACOUSTICS. 

Therefore  when  an  echo  repeats  seven  syllables,  the  reflecting 
surface  is  1,142  feet  distant;  for  sound  travels  at  the  rate  of 
1,142  feet  per  second,  and  the  distance  from  the  speaker  to  the 
reflecting  object,  and  ^again  from  the  latter  to  the  former,  is 
twice  1,142  feet.  When  the  echo  returns  14  syllables,  the  re- 
flecting object  must  be  2,284  feet  distant,  and  so  on. 

709.  It  is  stated  that  a  famous  echo  in  Woodstock,  (Eng- 
land,) repeats  seventeen  syllables  in  the  day,  and  twenty  in  the 
night,  and  en  the  north  side  of  Shepley  church  in  Sussex,  it  is 
said  that  an  echo  repeats  distinctly,  under  favorable  circum- 
stances, twenty-one  syllables. 

710.  Effects  of  Surface. — On  a  smooth  surface,  the  rays,  or 
pulses  of  sound,  will  pass  with  less  impediment  than  on  a  rough 
one.     For  this  reason,  persons  can  talk  to  each  other  on  the 
opposite  sides  of  a  river,  when  they  could  not  be  understood  at 
the  same  distance  over  the  land.     The  report  of  a  cannon  at 
sea,  when  the  water  is  smooth,  may  be  heard  at  a  great  dis- 
tance, but  if  the  sea  is  rough,  even  without  wind,  the  sound  will 
be  broken,  and  will  reach  only  half  as  far. 

711.  MUSICAL  INSTRUMENTS. — The  strings  of  musical  instru- 
ments are  elastic  cords,  which  being  fixed  at  each  end,  produce 
sounds  by  vibrating  in  the  middle. 

The  string  of  a  violin  or  piano,  when  pulled  to  one  side  by 
its  middle,  and  let  go,  vibrates  backward  and  forward,  like  a 
pendulum,  and  striking  rapidly  against  the  air,  produces  tones, 
which  are  grave,  or  acute,  according  to  its  tension,  size,  or 
length. 

712.  The  manner  FIG-  1GO- 

in    which     such    a  ^^~— — - 

string      vibrates     is          ^^^-~~~ • 
shown  by  Fig.  160. 

If  pulled  from  E 
to  A,  it  will  not  stop 
again  at  E,  but  in  Musical  string. 

passing    from  A  to 

E,  it  will  gain  a  momentum,  which  will  carry  it  to  C,  and  in 
returning,  its  momentum  will  again  carry  it  to  D,  and  so  on, 
backward  arid  forward,  like  a  pendulum,  until  its  tension,  arid 
the  resistance  of  the  air,  will  finally  bring  it  to  rest. 

713.  Tones  depend  on   Size  and   Tension. — The  grave  or 

709.  How  many  syllables  is  it  said  some  echoes  repeat  ?  710.  Why  is  it  that  per- 
eons  can  converse  on  the  opposite  sides  of  a  river,  when  they  could  not  hear  each 
other  at  the  same  distance  over  the  land  7  711.  How  do  the  strings  of  musical  instru- 
ments produce  sounds  7  712.  Explain  Fig.  160. 


ACOUSTICS.  205 

sharp  tones  of  the  same  string,  depend  on  its  different  degrees 
of  tension  ;  hence,  if  a  string  be  struck,  and  while  vibrating,  its 
tension  be  increased,  its  tone  will  be  changed  from  a  lower  to  a 
higher  pitch. 

Strings  of  the  same  length'are  made  to  vibrate  slow,  or  quick, 
and  consequently  to  produce  a  variety  of  sounds,  by  making 
some  larger  than  others,  and  giving  them  different  degrees  of 
tension.  The  violin  and  bass  viol  are  familiar  examples  of  this. 
The  low,  or  bass  strings,  are  covered  with  metallic  wire,  in  order 
to  make  their  magnitude  and  weight  prevent  their  vibration 
from  being  too  rapid,  and  thus  they  are  made  to  give  deep  or 
grave  tones.  The  other  strings  are  diminished  in  thickness,  and 
increased  in  tension,  so  as  to  make  them  produce  a  greater 
number  of  vibrations  in  a  given  time,  and  thus  their  tones  be- 
come sharp  or  acute  in  proportion. 

714.  JEoiJAx  HARP. — Under  certain  circumstances,  a  long 
string  will  divide  itself  into  halves,  thirds,  or  quarters,  without 
depressing  any  part  of  it,  and  thus  give  several  harmonious 
tones  at  the  same  time. 

The  fairy  tones  of  the  u^Eolian  harp  are  produced  in  this  man- 
ner. This  instrument  consists  of  a  simple  box  of  wood,  with 
four  or  five  strings,  two  or  three  feet  long,  fastened  at  each  end. 
These  are  tuned  in  unison,  so  that  when  made  to  vibrate  with 
force,  they  produce  the  same  tones.  But  when  suspended  in  a 
gentle  breeze,  each  string,  according  to  the  manner  or  force  in 
which  it  receives  the  blast,  either  sounds,  as  a  whole,  of  is 
divided  into  several  parts,  as  above  described.  The  result 
of  which,  is  the  production  of  the  most  pleasing  combination 
and  succession  of  sounds,  that  the  ear  ever  listened  to  or  fancv 
perhaps  conceived.  After  a  pause,  this  fairy  harp  is  often  heard 
beginning  with  a  low  and  solemn  note,  like  the  bass  of  distant 
music  in  the  sky ;  the  sound  then  swells  as  if  approaching,  and 
other  tones  break  forth,  mingling  with  the  first,  and  with  each 
other. 

715.  The  manner  in  which  a  string  vibrates  in  parts,  will  be 
understood  by  Fig.  161. 

Suppose  the  whole  length  of  the  string  to  be  from  A  to  B, 
and  that  it  is  fixed  at  these  two  points.  The  portion  from  B  to 
C  vibrates  as  though  it  was  fixed  at  -C,  and  its  tone  differs  from 
those  of  the  other  parts  of  the  string.  The  same  happens  from 

713.  On  what  do  the  grave  or  acute  tones  of  the  same  strin?  depend  ?  Why  ate 
the  ba=s  strings  ofmstruments  covered  with  metallic  wire  ?  714.  Why  is  there  a  va* 
riety  of  tones  in  the  ^Eolian  harp,  since  ail  the  strings  are  tuned  in  unison  1  715.  Ex- 
plain Fig  161s  showing  the  manner  in  which  strings  vibrate  in  parts. 


206 


ACOUSTICS. 


C  to  D,  and  from  D  to  A.  While  a  string  is  thus  vibrating,  if 
a  small  piece  of  paper  be  laid  on  the  part  C,  or  D,  it  will  re- 
main, but -if  plactd  on  any  other  part  of  the  string,  it  will  bo 
shaken  off. 


FIG.  161. 


JEolian  Harp. 

716.  MONOCHORD. — An  instrument  called monocJwrd  "single 
string,"  or  sonometer  "  sound  measurer,"  is  used  to  determine 
the  number  and  theory  of  musical  vibrations,  as  applied  to 
stringed  instruments.  It  consists  of  a  wooden  box,  several  feet 
in  length,  1,  2,  Fig.  162.  At  A,  a  catgut  or  metallic  string  is 
fastened,  which  passing  over  the  bridges  B  and  C,  and  then 
over  the  roller  D,  has  a  weight  suspended  for  its  tension  at  E. 


FIG.  162. 


Monochord. 


The  bridge  C  is  attached  to  a  scale,  on  which  it  moves, 
so  that  the  string  can  be  shortened  at  pleasure.  There  is  also 
provided  a  number  of  leaden  weights,  having  slits  to  the  center, 
to  be  slipped  on  the  string,  and  by  which  its  tension  can  be  in- 
creased or  diminished. 

717.  By  means  of  the  monochord,  many  curious  and  important 
inferences,  with  respect  to  stringed  instruments  have  been  drawn. 
We  extract  from  Muller  a  few  of  the  most  important  of  these 
laws. 


716.  What  is  the  meaning  of  monochord,  and  what  its  use?    Describe  the  mono 
chord  * 


WIND    INSTRUMENTS.  207 

718.  The  number  of  vibrations  of  a  string,  is  inversely  as  its 
length. 

If  the  string  of  any  instrument  makes  a  given  number  of  vi- 
brations in  a  certain  time,  it  would  make  in  the  same  time,  2, 
3,  or  4  times  as  many  vibrations,  if,  with  the  same  tension,  we 
let  only  £,  i,  or  -J-  of  its  length  vibrate,  and  so  in  these  propor- 
tions, whether  it  be  made  longer  or  shorter. 

719.  The  number  of  vibrations  of  a  string  is  proportional  to 
the  square  root  of  its  stretching  weight,  or  its  tension. 

Thus,  if  the  tension  of  a  given  length  of  string  be  equal  to  4, 
9,  or  16,  then  the  velocity  of  its  vibrations  will  be  2,  3,  or  4 
times  as  great. 

720.  The  number  of  vibrations  of  different  strings,  of  the 
same  substance,  is  inversely  as  their  thickness. 

If  we  take  two  steel  wires  of  equal  length,  whose  diameters 
are  as  1  and  2,  then  with  the  same  tension,  1  will  make  twice 
as  many  vibrations  as  2  in  the  same  time. 

721.  Capacity  of  the  Human  Ear. — From  Prof.  Hoblyn's, 
London,  edition  of  this  work,  we  add  the  following :    "  The  ca- 
pacity of  the  human  ear  for  appreciating  the  vibrations  of  a  son- 
orous body,  is  restricted  within  certain  limits.     It  has  been 
proved  by  experiment,  that  the  lowest  note  we  are  capable  of 
perceiving,  is  that  produced  by  a  body  performing  32  half  vi- 
brations, or  16  impulses,  in  one  second  of  time ;  and  the  highest, 
that  which  is  performed  by  16,000  impulses  in  the  same  time. 
It  is  stated,  however,  that  a  finely  attuned  ear  is  capable  of  ap- 
preciating, as  a  distinct  sound,  a  kind  of  hissing  noise,  occasioned 
by  48,000  half  vibrations,  or  24,000  impulses  in  a  second  of 
time." 


WIND    INSTRUMENTS. 


722.  In  stringed  instruments,  we  have  seen  that  the  sounds 
are  produced  by  the  vibration  of  stretched  cords  on  the  air. 

In  musical  pipes  the  tones  are  produced,  in  part,  by  the  pass- 
age of  the  air  through  apertures  of  various  forms,  and  in  part 
by  the  vibration  of  the  pipes  themselves. 

723.  Organ  Pipes. — The  most  complicated,  important,  and 
costly  instrument  is  the  organ.     This,  indeed,  embraces  in  its 
structure  nearly  every  known  wind  instrument,  and  therefore 
may  be  considered  as  a  collection  of  such  instruments,  each  of 

718.  To  what  is  the  number  of  vibrationsof  a  string  proportioned  ?  719.  How  docs 
tension  affect  the  vibrations  1  720.  How  does  thickness  affect  vibrations?  721.  What 
is  said  of  the  capacity  of  the  ear  to  appreciate  sounds?  722.  How  are  the  tones  in 
musical  pipes  produced  ?  723.  What  instruments  does  the  organ  embrace? 


208 


WIND    INSTRUMENTS. 


FIG.  163. 


which  may  be  played  separately,  or,  when  great  power  is  re- 
quired, several  may  be  played  in  unison. 

724.  Stops. — A  stop  consists  of  a  rank  of  pipes  on  a  uniform 
model.     Some  are  only  treble,  and  others  only  bass  stops.     In 
general,  however,  a  stop  includes  the  pipes  belonging  to  each 
instrument,   as  the  Flute,  Trumpet,  Hautboy,  and'Dulciana 
stops. 

725.  The  Diapason  (which  means  through  all) 
is  the  principal  stop,  and  on  this  all  the  other 
stops  are  founded,  or  are  made  to  correspond. 

726.  Flue  and  Reed  Stops. — This  is  the  great 
division  of  the  whole  organ,  and  depends  on  the 
mechanism  by  which  the  tones   are  produced, 
every  organ   in  this   respect,   having  only  two 
stops,  or  sorts  of  pipes,  however  numerous  the 
individual  stops  may  be. 

727.  flue  Pipes. — These  consist  of  the  body 
or  tube  B,  Fig.  163,  and  the  foot  P,  between 
which  there  is  a  diaphragm  or  partition,  having 
a  narrow,  transverse  aperture  to  emit  the  wind 
from  the  bellows,  as  shown  by  the  figure.     Over 
this  aperture  is  a  sharp  edge  called  the  upper  lip, 
against  which  the  wind  is  forced,  and  by  which 
the  sound  is  produced,  and  which  is"  modified  by 
the  size  and  form  of  the  pipe. 

728.  Chestnut  Whistle. — The  chestnut  or  wil- 
low whistle,  made  by  every  lad  in  the  country,  is 
a  good  illustration  of  the  flue  organ  pipe,  the 
construction  of  both,  being  precisely  on  the  same 
principle. 

729.  Reed  Pipes. — These  differ  from  the  above, 
in  having  a  piece  of  thin  brass,  or  other  metal, 
placed  in  the  mouth  of  the  pipe,  and  called  the 
tongue,  the  vibration  of  which  produces  the  sound. 
The  tongue  is  fastened  to  a  cylindrical  piece  of 
metal  between  C  C,  Fig.  164,  which  is  called  the 
block.     The  dotted  lines  C  C,  show  the  tuning 
wire,  which  passes  through  the  block,  and  by  the 

sliding  of  which,  up  and  down,  the  tones   are       Reed  Pipe. 


Flue  Pipe. 


FIG.  164. 


724.  What  is  meant  by  a  stop  ?  725.  What  is  said  of  the  diapason  stop  ?  726.  Into 
what  stops  is  the  entire  organ  divided?  727.  Show  by  Fig.  163,  the  construction  of 
the  flue  pipes.  728.  What  is  said  to  be  a  good  illustration  of  these  pipes  1  729.  De- 
scribe the  parts  of  a  reed  pipe  by  Fig.  164. 


WIND    INSTRUMENTS.  200 

varied,  the  pitch  becoming  flat  or  sharp,  as  the  tongue  is  made 
long  or  short. 

the  reed  pipes  are  generally  of  metal,  the  body  of  which  is 
shown  by  A  B. 

730.  Structure  of  the  Pipes. — The  large  pipes  are  commonly 
made  of  wood,  and  are  square  in  form,  though  some  wood 
pipes  are  only  a  few  inches  long.     The  largest  of  these  pipes 
are  32  feet  long  and  15  inches  in  diameter. 

731.  The  metal  pipes  are  in  the  form  of  a  cone  or  cylinder,  most 
of  the  smaller  ones  being  of  these  forms  and  substance.     In  a 
few  instances,  metallic  pipes  of  immense  size  and  weight  have 
been  constructed. 

The  largest  ever  made,  is  at  Birmingham,  England,  which 
is  32  feet  Jong  and  24  inches  in  diameter.  It  is  of  zinc,  in 
form  of  a  cylinder,  standing  in  front  of  all  the  other  pipes. 

732.  Tuning  the   Organ.— The  pipes  are  tuned  by  various 
means,  depending  on  frheir  forms ;  the  substance  of  which  they 
are  made,  and  whether  they  are  open  or  stopped. 

733.  Stopped  wooden  pipes  are  tuned  by  a  pompion,  or  stop- 
per, which  is  of  wood,  covered  with  leather,  exactly  fitting  the 
end,  and  which  is  drawn  up,  or  pushed  down,  to  make  the  tones 
more  grave  or  sharp. 

The  stopped  metal  pipes,  have  a  cap  on  the  top,  and  by  the 
movement  of  which,  they  are  tuned  on  the  same  principle  as 
those  of  wood.  In  some  cases,  stopped  metallic  pipes  are  tuned 
by  means  of  ears  on  each  side  of  the  mouth,  by  the  bending  of 
which,  the  tones  are  varied. 

Open  metal  pipes  are  tuned  by  a  wooden  instrument,  one 
end  of  which  is  a  solid  cone*  and  the  other  a  hollow  cone.  By 
this,  the  tops  of  the  pipes  are  expanded  by  introducing  the  solid 
end,  to  make  the .  pitch  sharper,  and-  contracted  by  the  hollow 
cone,  to  make  the  pitch  fatter. 

The  reed  pipes,  as  already  noticed,  are  tuned  by  the  motions 
of  the  tuning  wire. 

Reed  Pipes  vary  with  the  Temperature. — The  tongues  of 
these  pipes  vary  in  length  by  heat  and  cold,  and  hence  their 
tones  change,  for  the  same  reason  that  the  clock  goes  faster  in 
winter  than  in  summer,  as  explained,  283.  It  is  probably  on 
this  account  that  organists  find  difficulty  in  keeping  these  stops 
in  tune. 

730.  What  is  said  of  the  structure  of  these  pipes  ?  731.  Of  what  size  are  the  largest 
organ  pipes  1  732.  How  are  the  pipes  tuned  1  733.  How  are  the  stooped  nines  tuned  1 
How  are  the  open  p,Pes  tuned  ?  What  is  said  of  the  influence  ofTemperTuSe  on  the 


210  LARGE    ORGANS. 

734.  ANTIQUITY  OF  THE  ORGAN. — The  earliest  account  of 
any  instrument  similar  to  the  organ,  occurs  in  the  Tenth  Book 
of  Vitruvius,  a  Greek  writer,  who  lived  a  century  before  the 
Christian  era.  This  was  moved  by  water,  and  hence  was  called 
a  hydraulicon. 

The  first  organ  spoken  of  in  France,  was  of  Greek  construc- 
tion, and  was  sent  to  King  Pepin,  the  father  of  Charlemagne, 
by  the  Emperor  Constantine,  about  A.  D.  757.  This  was 
moved  by  wind. 

The  first  of  any  size  known  in  England,  was  that  of  Winches- 
ter Cathedra],  in  951.  This  had  26  pairs  of  bellows,  which  it 
required  70  men  to  work.  It  had  10  keys,  and  40  pipes  to 
each  key. 

Notwithstanding  the  antiquity  of  this  invention,  it  was  not 
until  after  the  Reformation  that  any  great  improvements  were 
made  in  this  instrument.  Even  so  late  as  1660,  only  four  organ 
builders  were  to  be  found  in  Great  Britain. 

This  instrument,  in  our  country,  was  unknown  to  the  common 
people  a  century  ago ;  and  at  the  time  of  our  revolution,  com- 
paratively few  persons,  except  in  large  cities,  had  ever  heard  an 
organ.  It  is  hardly  necessary  to  add,  that  the  organ,  as  it  now 
exists,  is  an  entirely  different  instrument  from  that  so  called 
only  fifty  years  ago,  and  that,  at  present,  no  village  having  a 
church  of  any  pretensions,  is  without  an  organ. 


LARGE  ORGANS. 


735.  Perhaps  we  can  not  gratify  our  readers  more  than  to 
add  short  notices  of  a  few  of  the  largest  organs  in  the  world. 

Haarlem  Organ. — This  has  long  been  the  most  celebrated  of 
organs.  It  was  built  in  1638,  at  the  cost  of  $60,000.  The  case 
is  108  feet  high  by  50  feet  wide.  It  has  60  stops ;  12  pair  of 
bellows;  4  rows  of  keys;  5000  pipes,  of  which  two  are  32  feet 
long  and  15  inches  wide.  The  fee  for  hearing  the  whole  is  $5. 

Freyburg  Organ,  in  Switzerland. — It  is  said  that  no  instru- 
ment ever  was,  or  ever  will  be  built  like  this ;  the  artist,  Moser, 
refusing  to  build  another,  and  no  one  being  allowed  to  see  the 
interior.  The  wonder,  and  the  secret,  with  respect  to  this  organ, 
is  in  its  having  a  stop,  the  tones  of  which  are  so  exactly  like 
those  of  the.  human  voice,  that  visitors  mistake  it  for  a  large  choir 
of  singers.  It  has  68  stops  and  4  rows  of  keys. 

Music  Hall,  Edinburgh,  Organ. — This  immense  instrument 

734.  What  is  said  of  the  antiquity  of  the  organ  7 


HARMONICON. 


211 


has  82  stops,  4  rows  of  keys,  1  wooden  pipe  of  32  feet,  and 
several  of  metal  of  16  feet  in  length. 

Hamburgh  Organ. — This  organ,  in  St.  Michael's  Church,  was 
biiilt  in  1762,  and  cost  more  than  $20,000.  It  has  4  rows  of 
keys;  three  pipes  of  32  feet,  and  nine  of  16  feet;  10  wind 
chests  and  10  pair  of  bellows.  The  pipes  of  the  large  pedal 
stop,  are  of  pure  tin  highly  polished,  and  placed  in  front 

The  Weingarten  Organ. — This  is  in  the  Benedictine  Monas- 
tery, in  Suabia,  and  was  built  about  1750.  It  has  4  rows  of 
keys ;  3  pipes  of  32  feet;  4  of  16  feet,  and  4  unisons.  It  has 
in  the  whole,  6,666  pipes ;  namely,  in  the  great  organ,  2,176 ; 
in  the  choir,  1,176  ;  in  the  third  organ,  1,274 ;  in  the  echo  or- 
gan, 1,225,  and  in  the  pedal  organ,  815. 

Berlin  Organ. — This  organ,  at  Berlin,  Prussia,  was  designed 
to  be  the  largest  in  the  world,  and  to  contain  150  stops  and  6 
rows  of  keys,  besides  the  pedals,  but  it  remains  unfinished. 

Baltimore  Cathedral  Organ. — This  is  said  to  be  the  largest 
in  the  United  States.  It  has  36  stops  and  2,213  pipes,  the 
largest  being  32  feet  long. 

HARMONICON. 

This  is  a  musical  instrument  invented  by  Dr.  Franklin,  though 
i*  has  been  much  improved  since  his  day. 

FIG.  165. 


©   ©  o  © 
O  ©  O  © 


Harmonicon. 


It  consists  of  a  number  of  glass  goblets  of  different  sizes,  and 
so  attuned  to  each  other  as  to  form  the  harmonic  scale. 


212  WIND. 

*• 

They  are  firmly  fastened  to  the  bottom  of  a  box,  their  tones 
being  so  nicely  adapted  to  the  scale,  by  the  artist  who  con- 
structs them,  as  to  need  no  tuning,  though  one  or  two  of  them 
contain  water  as  a  convenience. 

They  are  played  by  touching  the  edges  with  the  wet  finger, 
and  their  tones  may  be  prolonged,  and  made  to  swell  or  dimin- 
ish, like  .those  of  the  violin. 

Perhaps  no  music  to  which  the  human  ear  has  ever  listened, 
is  equal  in  sweetness,  delicacy,  and  smoothness  to  this.  No  one 
can  hear  it  without  a  thrill  of  delight,  nor  for  the  first  time, 
without  astonishment.  It  is  indeed  an  .JSolian  harp  under 
command  of  the  artist. 

The  arrangement  and  comparative  sizes  of  the  goblets,  are 
shown  by  Fig.  165,  which  presents  the  natural  key,  or  C  major. 

The  goblets  hold  from  a  quart  to  half  a  pint,  and  their  tones 
depend,  in  part,  upon  their  capacity,  and  in  part  upon  the 
weight  or  thickness. 

The  instrument  here  represented,  is  capable  of  producing  all 
the  tones  of  the  most  common  and  simple  melodies. 

We  are  told  that  Mr.  Francis  H.  Smith,  of  Baltimore,  furn- 
ishes Harmonicons,  put  up  in  boxes,  at  various  prices,  from  18 
to  85  dollars. 

ATMOSPHERIC    PHENOMENA. 

736.  The  term  atmosphere  is  from  two  Greek  words,  which 
signify  vapor  and  sphere.     It  is  the  air  which  surrounds  the 
earth  to  the  height  of  forty-five  miles,  and  is  essential  to  the 
lives  of  all  animals,  and  the  production  of  all  vegetables. 

All  meteorological  phenomena,  with  which  we  are  acquainted, 
depend  chiefly,  if  not  entirely,  on  the  influence  of  the  atmos- 
phere. Fogs,  winds,  rain,  dew,  hail,  snow,  thunder,  lightning, 
electricity,  sound,  and  a  variety  of  other  phenomena  of  daily 
occurrence,  belong  to  the  atmosphere.  We  have,  however,  only 
room  for  the  most  common  result  of  atmospheric  changes. 
Wind  and  Rain. 

WIND 

737.  Wind  is  nothing  more  than^  air  in  motion.     The  use 
of  a  fan,  in  warm  weather,  only  serves  to  move  the  air,  and  thus 
to  make  a  little  breeze  about  the  person  using  it. 

736.  What  is  the  atmosphere?  How  high  does  the  atmosphere  extend?  What 
phenomena  mentioned,  depend  on  the  atmosphere?  737.  What  is  wind?  As  a 
natural  phenomenon,  how  is  wind  produced  ;  or,  what  is  the  cause  of  wind?  How 
is  this  illustrated  7 


WIND.  213 

As  a  natural  phenomenon,  that  motion  of  the  air  which  we 
call  wind,  is  produced  in  consequence  of  there  being  a  greater 
degree  of  heat  in  one  place  than  in  another.  The  air  thus 
heated,  rises  upward,  while  that  which  surrounds  this,  moves 
forward  to  restore  the  equilibrium. 

The  truth  of  this  is  illustrated  by  the  fact,  that  during  the 
burning  of  a  house  in  a  calm  night,  the  motion  of  the  air  to- 
ward the  place  where  it  is  thus  rarefied,  makes  the  wind  blow 
from  every  point  toward  the  flame. 

738.  Sea  and  Land  Breeze. — On  islands,  situated  in  hot 
climates,  this  principle  is  charmingly  illustrated.     The  land, 
during  the  day  time,  being  under  the  rays  of  a  tropical  sun, 
becomes  heated  in  a  greater  degree  than  the  surrounding  ocean, 
and,  consequently,  there  rises  from  the  land  a  stream  of  warm 
air,  during  the  day,  while  the  cooler  air  from  the  surface  of  the 
water,  moving  forward  to  supply  this  partial  vacancy,  produces 
a  cool  breeze  setting  inland  on  all  sides  of  the  island.     This 
constitutes  the  sea  breeze,  which  is  so  delightful  to  the  inhabit- 
ants of  those  hot  countries,  and  without  which  men  could  hardly 
exist  in  some  of  the  most  luxuriant  islands  between  the  tropics. 

During  the  night,  the  motion  of  the  air  is  reversed,  because 
the  earth  being  heated  superficially,  soon  cools  when  the  sun  ia 
absent,  while  the  water,  being  warmed  several  feet  below  its 
surface,  retains  its  heat  longer. 

Consequently,  toward  morning,  the  earth  becomes  colder  than 
the  water,  and  the  air  sinking  down  upon  it,  seeks  an  equilib- 
rium, by  flowing  outward,  like  rays  from  a  center,  and  thus 
the  land  breeze  is  produced. 

The  wind  then  continues  to  blow  from  the  land  until  the 
equilibrium  is  restored,  or  until  the  morning  sun  makes  the  land 
of  the  same  temperature  as  the  water,  when  for  a  time  there 
will  be  a  dead  calm.  Then  again  the  land  becoming  warmer 
than  the  water,  the  sea  breeze  returns  as  before,  and  thus  the 
inhabitants  of  those  sultry  climates  are  constantly  refreshed  dur- 
ing the  summer  season,  with  alternate  land  and  sea  breezes. 

739.  TRADE  WINDS. — At  the  equator,  which  is  a  part  of  the 
earth  continually  under  the  heat  of  a  burning  sun,  the  air  is 
expanded,  and  ascends  upward,  so  as  to  produce  currents  from 
the  north  and  south,  which  move  forward  to  supply  the  place 
of  the  heated  air  as  it  rises. 

738.  In  the  islands  of  hot  climates,  why  does  the  wind  blow  inland  during  the  day 
and  off  the  land  during  the  night  ?  What  are  these  breezes  called  ?  739.  What  is 
said  of  the  ascent  of  heated  air  at  the  equator  ?  What  is  the  consequence  on  the  air 
toward  the  north  and  south  7  How  are  the  trade  winds  formed  1 


214  WIND. 

These  two  currents,  coming  from  latitudes  where  the  daily- 
motion  of  the  earth  is  less  than  at  the  equator,  do  not  obtain 
its  full  rate  of  motion,  and  therefore,  when  they  approach  the 
equator,  do  not  move  so  fast  eastward  as  that  portion  of  the 
earth,  by  the  difference  between  the  equator's  velocity,  and  that 
of  the  latitudes  from  which  they  come.  This  wind,  therefore, 
falls  behind  the  earth  in  her  diurnal  motion,  and  consequently 
has  a  relative  motion  toward  the  west.  This  constant  breeze 
toward  the  west  is  called  the  trade  wind,  because  a  large  por- 
tion of  the  commerce  of  nations  comes  within  its  influence. 

740.  Counter  Currents. — While  the  air  in  the  lower  regions 
of  the  atmosphere  is  thus  constantly  flowing  from  the  north  and 
south  toward  the  equator,  and  forming  the  trade  winds  between 
the  tropics,  the  heated  air  from  these  regions  as  perpetually 
rises,  and  forms  a  counter  current  through  the  higher  regions, 
toward  the  north  and  south  from  the  tropics,  thus  restoring  the 
equilibrium. 

This  counter  motion  of  the  air  in  the  upper  and  lower  regions 
is  illustrated  by  a  very  simple  experiment.  Open  a  door  a  few 
inches,  leading  into  a  heated  room,  and  hold  a  lighted  candle 
at  the  top  of  the  passage ;  the  current  of  air,  as  indicated  by 
the  direction  of  the  flame,  will  be  out  of  the  room.  Then  set 
the  candle  on  the  floor,  and  it  will  sjiow  that  the  current  is 
there  into  the  room.  Thus,  while  the  heated  air  rises  and  passes 
out  of  the  room,  at  the  same  time  that  which  is  colder  flows  in, 
along  the  floor,  to  take  its  place. 

This  explains  the  reason  why  our  feet  are  apt  to  suffer  with 
the  cold,  in  a  room  moderately  heated,  while  the  other  parts  of 
the  body  are  comfortable.  It  also  explains  why  those  who  sit 
in  the  gallery  of  a  church  are  sufficiently  warm,  while  those 
who  sit  below  may  be  shivering  with  the  cold. 

741.  From  such  facts,  showing  the  tendency  of  heated  air  to 
ascend,  while  that  which  is  colder  moves  forward  to  supply  its 
place,  it  is  easy  to  account  for  the  reason  why  the  wind  blows 
perpetually  from  the  north  and  south  toward  the  tropics ;  for 
the  air  being  heated,  as  stated  above,  it  ascends,  and  then  flows 
north  and  south  toward  the  poles,  until,  growing  cold,  it  sinks 
down  and  again  flows  toward  the  equator. 


740.  While  the  air  in  the  lower  regions  flows  from  the  north  and  south  toward  the 
equator,  in  what  direction  does  it  flow  in  the  higher  regions  ?  How  is  this  counter 
current  in  lower  and  upper  regions  illustrated  by  a  simple  experiment?  741.  What 
common  fact  does  this  experiment  illustrate?  Explain  Fig.  166,  and  show  which 
way  the  air  passes. 


WIND.  215 


Perhaps  these  opposite  motions  of  the  two 
better  understood  by  the  sketch,  Fig.  166. 


Opposite  Currents  of  Air. 

Suppose  A  B  0  to  represent  a  portion  of  the  earth's  surface, 
A  being  toward  the  north  pole,  C  toward  the  south  pole,  and 
B  the  equator.  The  currents  of  air  are  supposed  to  pass  in  the 
direction  of  the  arrows.  The  wind,  therefore,  from  A  to  B 
would  blow  on  the  surface  of  the  earth,  from  north  to  south, 
while  from  E  to  A,  the  upper  current  would  pass  from  south  to 
north,  until  it  came  to  A,  when  it  would  change  its  direction 
toward  the  south.  The  currents  in  the  southern  hemisphere 
being  governed  by  the  same  laws,  would  assume  similar  di- 
rections. 

VELOCITY    OF    WIND. 

742.  The  velocity  of  aerial  movements  amount,  according  to 
authors,  from  0  to  upwards  of  100  miles  an  hour ;  but  the  max- 
imum is  variously  stated  by  different  experiments,  nor  do  we 
see  how  any  great  degree  of  accuracy  can  be  attained  on  this 
point.     The  best  method  is,  to  deduce  the  velocity,  by  the  force 
of  wind  ;  which  is  done  by  an  instrument  invented  for  that  pur- 
pose by  Dr.  Lind,  a  figure  of  which  we  here  insert. 

743.  ANEMOMETER,  OR  WIND  MEASURER. — It  consists  of  a 
glass  tube,  Fig.  167,  bent  into  the  form  of  the  letter  U,  and 
open  at  both  ends.     The  upper  end  of  B  is  bent  to  the  hori- 
zontal direction,  and  is  widened  at  the  mouth  for  the  purpose 
of  receiving  the  wind.     The  tube  being  partly  filled  with  water, 
and  exposed  to  a  current  of  air,  the  fluid  is  depressed  in  that, 
and  of  course  rises  in  the  other  leg  of  the  tube.     As  the  water 


743.  What  is  the  name  of  the  instrument  which  measures  the  force  of  wind?    How 
is  it  constructed  ? 


216 


RAIN. 


is  on  a  level  in  both  branches  when  the  air  is  FIG.  167. 

still,  if  it  is  depressed  to  B  on  one  side,  it 
must  rise  to  C  on  the  other,  the  amount  of 
rise,  and  consequently  the  degree  of  force,  be- 
ing measured  by  a  graduated  scale.  Now  as 
the  pressure  of  water  is  as  its  height,  the  rise 
in  the  tube  will  not  be  in  direct  proportion  to 
the  force  of  the  wind,  but  the  velocity  of  the 
wind  will  be  in  the  ratio  of  the  square  root  of 
the  resistance.  The  tube  is  diminished  at  the 
base  to  check  the  undulations  of  the  water. 

744.  By  this  instrument  it  is  found  that  Anemometer. 

'  the  following  popular  expressions  with  respect 
to  aerial  currents,  are  indicated  on  the  scale  as  here  expressed. 

Velocity  of  the  Wind  in  miles  per  hour.  Com.  appellation  of  the  force  of  Wind. 

1 .  Hardly  perceptible. 

4 Gentle  breeze. 

6 Pleasant  Wind. 

10 Brisk  wind. 

15 Very  brisk  wind. 

20 High  wind. 

30 Very  high  wind. 

40 A  storm. 

50 A  hard  storm. 

60 A  great  storm. 

80 A  hurricane. 

100 A  violent  hurricane. 

RAIN. 

745.  Rain  is  falling  water  in  the  form  of  drops.  It  appears 
to  result  from  the  meeting  of  two  clouds  of  different  tempera- 
tures. 

In  explaining  the  theory  of  rain,  it  must  be  understood,  that 
warm  air  has  a  greater  capacity  for  moisture  than  cold.  It  is 
also  ascertained,  that  this  capacity  increases  at  a  much  faster 
ratio  than  the  increase  of  temperature  itself,  and  h'ence  it  fol- 
lows that  if  two  clouds  at  different  temperatures,  completely 
saturated,  meet  and  mingle  together,  a  precipitation  of  moisture 


744.  What  correspondence  is  there  between  the  velocity  of  wind,  and  common  ex- 
pressions? 745.  What  is  rain  ?  What  is  said  of  the  ratio  of  capacity  for  moisture 
increasing  faster  than  the  temperature  in  clouds?  Explain  the  reason  why,  when 
two  clouds  meet  of  different  temperatures,  rain  is  the  result. 


RAIN.  £17 

must  take  place  in  consequence  of  the  mixture.  This  would 
result  from  the  fact  that  the  warmest  cloud  contained  a  greater 
portion  of  moisture  than  is  indicated  by  its  temperature,  as 
stated  above,  while  the  mixture  would  form  a  mean  tempera- 
ture, but  the  mean  quantity  of  vapor  could  not  be  retained, 
since  the  sum  of  their  capacities  for  vapor  would  thus  be  di- 
minished. 

746.  Suppose  for  example,  that  at  the  temperature  of  15  de- 
grees, air  can  hold  200  parts  of  moisture ;  then  at  30  degrees 
it  would  hold  400  parts,  and  at  45  degrees  800  parts.     Now 
let  two  equal  bulks  of  this  air,  one  at  15,  and  the  other  at  45 
degrees  be  mixed,  the  compound  would  then  contain  200  and 
800  parts  of  moisture  =  1000,  that  is,  500  each,  and  the  tem- 
perature of  the  mixture  would  be  30  degrees.     But  at  this 
temperature,  air  is  saturated  with  400  parts  of  vapor,  therefore, 
100  parts  is  rejected  and  falls  in  the  form  of  rain. 

This  is  Dr.  Button's  theory  of  rain,  and  observation  has 
seemed  to  prove  its  truth. 

747.  RAIN    GAUGE. — This    is   an   instrument   designed   to 
measure  the  quantity  of  rain  which  falls  at  any  given  time  and 
place. 

748.  A  variety  of  forms,  some  quite  compli-          FIG.  J68. 
cated,  have  been  invented  for  this  purpose.     The 

most  simple  and  convenient,  for  common  pur- 
poses, is  that  represented  by  Fig.  168.  It  may 
be  two  feet  high,  round  in  form,  and  made  of 
tin,  or  copper,  well  painted.  It  is  furnished  with 
a  small  metallic  faucet  for  drawing  off  the  water, 
and  into  the  stem  of  this,  is  inserted  a  glass  tube, 
as  a  scale,  divided  into  inches  and  tenths  of 
inches.  This  may  be  done  by  means  of  paper, 
pasted  on  and  then  varnished. 

The  water  will  stand  at  the  same  height  in  the       Rain  Gause 
glass  scale  that  it  does  in  the  cylinder,  and  being 
on  the  outside,  the  quantity  may  be  known  at  a  glance.     If  the 
funnel,  or  top,  is  twice  the  size  of  the  cylinder,  then,  an  inch  in 
the  scale  will  indicate  half  an  inch  received  into  the  gauge,  or 
these   proportions  may  be   a  tenth,   when  much  accuracy  is 
required. 

746.  What  is  the  design  of  the  rain  ffau«rei "  747.  What  are  the  forms  and  materials 
of  this  instrumenl  1  748.  Describe  the  scale,  and  what  it  indicates  with  respect  to 
the  size  of  the  funnel  and  cylinder  ? 

10 


CHAPTER  XL 

OPTICS. 

1.  This  term,  derived  from  the  Greek,  signifies  seeing,  or  to 
see.     It  is  that  science  which  treats  of  vision,  and  the  laws, 
properties,  and  phenomena  of  light. 

2.  It  admits  of  two  divisions,  viz.,  Dioptrics,  or  a  discourse  on 
the  laws  of  refracted  light,  and  Catoptrics,  a  treatise  on  reflected 
light. 

This  science  involves  some  of  the  most  elegant  and  import- 
ant branches  of  natural  philosophy.  It  presents  us  with  exper- 
iments which  are  attractive  by  their  beauty,  and  which  astonish 
us  by  their  novelty ;  and,  at  the  same  time,  it  investigates  the 
principles  of  some  of  the  most  useful  among  the  articles  of 
common  life. 

3.  There  are  two  opinions  concerning  the  nature  of  light. 
Some  maintain  that  it  is  composed  of  material  particles,  which 
are  constantly  thrown  off  from  the  luminous  body  ;  while  others 
suppose  that  it  is  a  fluid,  diffused  through  all  nature,  and  that 
the  luminous,  or  burning  body,  occasions  waves  or  undulations 
in  this  fluid,  by  which  the  light  is  propagated  in  the  same  man- 
ner as  sound  is  conveyed  through  the  air. 

4.  The  most  probable  opinion,  however,  is  that  light  is  com- 
posed of  exceedingly  minute  particles  of  matter.     But  whatever 
may  be  the  nature  or  cause  of  light,  it  has  certain  general  prop- 
erties or  effects  which  we  can  investigate.     Thus,  by  experiment, 
we  can  determine  the  laws  by  which  it  is  governed  in  its  pass- 
age through  different  transparent  substances,  and  also  those  by 
which  it  is  governed  when  it  strikes  a  substance  through  which 
it  can  not  pass.     We  can  likewise  test  its  nature  to  a  certain 
degree,  by  decomposing  or  dividing  it  into  its  elementary  parts, 
as  the  chemist  decomposes  any  substance  he  wishes  to  analyze. 

5.  Definitions. — To  understand  the  science  of  optics,  it  is 
necessary  to  define  several  terms,  which,  although  some  of  them 
may  be  in  common  use,  have  a  technical  meaning,  when  ap- 
plied to  this  science. 

1.  What  is  the  meaning  of  optics?  2.  What  are  the  meaning  of  dioptrics  and  cat- 
optrics? What  is  said  of  the  elegance  and  importance  of  this  science?  3.  What  ar^ 
the  two  opinions  concerning  the  nature  of  light  1  4.  What  is  the  most  probable  opin- 
ion ?  5.  What  is  light  1 


OPTICS.  219 

Light  is  that  principle,  or  substance,  which  enables  us  to  see 
any  body  from  which  it  proceeds.  If  a  luminous  substance,  as 
a  burning  candle,  be  carried  into  a  dark  room,  the  objects  in 
the  room  become  visible,  because  they  reflect  the  light  of  the 
candle  to  our  eyes. 

6.  Luminous  bodies  are  such  as  emit  light  from  their  own 
substance.     The  sun,  fire,  and  phosphorus  are  luminous  bodies. 
The  moon,  and  the  other  planets,  are  not  luminous,  since  they 
borrow  their  light  from  the  sun. 

7.  Transparent  bodies  are  such  as  permit  the  rays  of  light  to 
pass  freely  through  them.     Air  and  some  of  the  gases  are  per- 
fectly transparent,  since  they  transmit  light  without  being  visible 
themselves.     Glass  and  water  are  also  considered  transparent, 
but  they  are  not  perfectly  so,  sines  they  are  themselves  visible, 
and  therefore  do  not  suffer  the  light  to  pass  through  them  with- 
out interruption. 

8.  Translucent  bodies  are  such  as  permit  the  light  to  pass, 
but  not  in  sufficient  quantity  to  render  objects  distinct,  when 
seen  through  them. 

9.  Opaque  is  the  reverse  of  transparent.     Any  body  which 
permits  none  of  the  rays  of  light  to  pass  through  it,  is  opaque. 

10.  Illuminated,    enlightened.      Any    thing   is    illuminated 
when  the  light  shines  upon  it  so  as  to  make  it  visible.     Every 
object  exposed  to  the  sun  is  illuminated.     A  lamp  illuminates  a 
room,  and  every  thing  in  it. 

A  Ray  is  a  single  line  of  light,  as  it  coirues  from  a  luminous 
body. 

A  Beam  of  light  is  a  body  of  parallel  rays. 

A  Pencil  of  light  is  a  body  of  diverging  or  converging  rays. 

Divergent  rays  are  such  as  come  from  a  point,  and  contin- 
ually separate  wider  apart  as  they  proceed. 

Convergent  rays  are  those  which  approach  each  other,  so  as 
to  meet  at  a  common  point. 

Luminous  bodies  emit  rays,  or  pencils  of  light,  in  every  di- 
rection, so  that  the  space  through  which  they  are  visible,  is 
filled  with  them  at  every  possible  point. 

Thus,  the  sun  illuminates  every  point  of  space,  within  the 
whole  solar  system.  A  light,  as  that  of  a  light-house,  which 

6.  What  is  a  luminous  body?  7.  What  is  a  transparent  body  ?  Are  glass  and  wa 
ter  perfectly  transparent  ?  How  is  it  proved  that  air  is  perfectly  transparent?  8 
What  are  trans  ucent  bodies?  9.  What  are  opaque  bodies?  10.  What  is  meant  by* 
illuminated  }  What  is  a  ray  of  light  ?  What  is  a  beam  .'  What  a  pencil  1  What  are 
divergent  rays?  What  are  convergent  rays  ?  In  what  direction  do  luminous  bodies 
emit  light  ?  How  is  it  proved  that  a  luminous  body  fills  every  point  within  a  certain 
distance  with  light  7 


220  OPTICS. 

can  be  seen  from  the  distance  of  ten  miles  in  one  direction,  fills 
every  point  in  a  circuit  of  ten  miles  from  it,  with  light.  Were 
this  not  the  case,  the  light  from  it  could  not  be  seen  from  every 
point  within  that  circumference. 

11.  Motion  of  light.  The  rays  of  lighj,  move  forward  in 
straight  lines  from  the  luminous  body,  and  are  never  turned  out 
of  their  course,  except  by  some  obstacle. 

FIG.  169. 


Motion  of  Light. 

Let  A,  Fig.  169,  be  a  beam  of  light  from  the  sun  passing 
through  a  small  orifice  in  the  window  shutter,  B.  The  sun 
can  not  be  seen  through  the  crooked  tube  C,  because  the  beam 
passing  in  a  straight  line,  strikes  the  side  of  the  tube,  and  there- 
fore does  not  pass  through  it. 

12.  All  illuminated  bodies,  wrhether  natural  or  artificial,  throw 
off  light  in  every  direction  of  the  same  color  as  themselves, 
though  the  light  with  which  they  are  illuminated  is  white  or 
without  color. 

This  fact  is  obvious  to  all  who  are  endowed  with  sight.  Thus 
the  light  proceeding  from  grass  is  green,  while  that  proceeding 
from  a  rose  is  red,  and  so  of  every  other  color. 

We  shall  be  convinced,  in  another  place,  that  the  white  light 
with  which  things  are  illuminated,  is  really  composed  of  several 
colors,  and  that  bodies  reflect  only  the  rays  of  their  own  color, 
while  they  absorb  all  the  other  rays. 

13.  Velocity  of  Light. — Light  moves  with  the  amazing  ra- 
pidity of  about  95  millions  of  miles  in  8-£  minutes,  since  it  is 
proved  by  certain  astronomical  observations,  that  the  light  of 
the  sun  comes  to  the  earth  in  that  time.     This  velocity  is  so 
great,  that  to  any  distance  at  which  an  artificial  light  can  be 
seen,  it  seems  to  be  transmitted  instantaneously. 

11.  Why  can  not  a  beam  of  light  be  seen  through  a  bent  tube  1  12.  What  is  the 
eolor  of  the  light  which  different  bodies  throw  off?  If  grass  throws  off  green  light, 
what  becomes  of  the  other  rays  7  13.  What  is  the  rate  of  velocity  with  which  light 
moves?  Can  we  perceive  any  difference  in  the  time  which  it  takes  an  artificial 
light  to  pass  to  u»  from  a  great  or  small  distance  ? 


REFRACTION    OF   LIGHT.  221 

If  a  ton  of  gunpowder  were  exploded  on  the  top  of  a  moun- 
tain, where  its  light  could  be  seen  a  hundred  miles,  no  percept- 
ible difference  would  be  observed  in  the  time  of  its  appearance 
on  the  spot,  and  at -the  distance  of  a  hundred  miles. 

DIOPTRICS,    OR    THE    REFRACTION    OF    LIGHT. 

14.  Although  a  ray  of  light  will  pass  in  a  straight  line,  when 
not  interrupted,  yet  when  it  passes  obliquely  from  one  transpar- 
ent body  into  another,  of  a  different  density,  it  leaves  its  linear 
direction,  and  is  bent,  or  refracted  more  or  less,  out  of  its  former 
course. 

This  change  in  the  direction  of  light,  FIG.  170. 

seems  to  arise  from  a  certain  power, 
or  quality,  which  transparent  bodies 
possess  in  different  degrees  ;  for  some 
substances  bend  the  rays  of  light  much 
more  obliquely  than  others. 

The  manner  in  which  the  rays  of 
light  are  refracted,  may  be  readily  un- 
derstood by  Fig.  170. 

Let  A  be  a  ray  of  the  sun's  light, 
proceeding  obliquely  toward  the  sur-  Refraction. 

face  of  the  water  C  D,  and  let  E  be 
the  point  which  it  would  strike,  if  moving  only  through  the  air. 
Now,  instead  of  passing  through  the  water  in  the  line  A  E,  it 
will  be  bent  or  refracted,  on  entering  the  water,  from  O  to 
N,  and  having  passed  through  the  fluid  it  is  again  refracted  in 
a  contrary  direction  on  passing  out  of  the  water,  and  then  pro- 
ceeds onward  in  a  straight  line  as  before. 

15.  Cup  and  Shilling. — The  refraction  of  water  is  beauti 
fully  proved  by  the  following  simple  experiment.     Place  an 
empty  cup,  Fig.  171,  with  a  shilling  on  the  bottom,  in  such  a 
position  that  the  side  of  the  cup  will  just  hide  the  piece  of 
money  from  the  eye.     Then  let  another  person  fill  the  cup  with 
water,  keeping  the  eye  in  the  same  position  as  before.     As  the 
water  is  poured  in,  the  shilling  will  become  visible,  appearing 
to  rise  with  the  water.  '  The  effect  of  the  water  is  to  bend  the 
ray  of  light  coming  from  the  shilling,  so  as  to  make  it  meet  the 
eye  below  the  point  where  it  otherwise  would.     Thus  the  eye 

14.  What  is  meant  by  the  refraction  of  light  7  Do  all  transparent  bodies  refract 
1'ght  equally?  Explain  Fig.  170,  and  show  how  the  ray  is  refracted  in  passing  into, 
and  out  of  the  water.  15.  Explain  Fig.  171,  and  state  the  reason  why  the  shilling 
seems  to  be  raised  up  by  pouring  in  the  water. 


222  REFRACTION    OF    LIGHT. 

could  not  see  the  shilling  in     ^J»  mo-  ITL 

the  direction  of  C,  since  the 
line  of  vision  toward  A  and 
C  is  hidden  by  the  side  of 
the  cup.  But  the  refraction 
of  the  water  bends  the  ray 
downward,-  producing  the 
same  effect  as  though  the  ob- 
ject had  been  raised  upward, 
and  hence  it  becomes  visible.  c 

1 6.  Refraction  by  Several  Cup  and  shilling. 

Media.  —  Any    transparent 

body  through  which  light  passes,  is  called  a  medium,  and  it  is 
found  in  all  cases,  "  that  where  a  ray  of  light  passes  obliquely 
from  one  medium  into  another  of  a  different  density,  it  is  re- 
fracted, or  turned  out  of  its  former  course"  This  is  illustrated 
in  the  above  examples,  the  water  being  a  more  dense  medium 
than  air.  The  refraction  takes  place  at  the  surface  of  the  me- 
dium, and  the  ray  is  refracted  in  its  passage  out  of  the  refract- 
ing substance  as  well  as  into  it. 

1*7.  If  the  ray,  after  having  passed  through'  the  water,  then 
strikes  upon  a  still  more  dense  medium,  as  a  pane  of  glass,  it 
will  again  be  refracted.  It  is  understood,  that  in  all  cases,  the 
ray  must  fall  upon  the  refracting  medium  obliquely,  in  order  to 
be  refracted,  for  if  it  proceeds  from  one  medium  to  another  per- 
pendicularly to  their  surfaces,  it  will  pass  straight  through  them 
all,  and  no  refraction  will  take  place. 

18.  Thus,  in  Fig.  172,  let  A  represent  air,  B  water,  and  C  a 
piece  of  glass.     The  ray  D,  striking  each  medium  in  a  perpen- 
dicular direction,  passes  through  them  all   in  a  straight  line. 
The  oblique  ray  passes  through  the  air  in  the  direction  of  C, 
but  meeting  the  water,  is  refracted  in  the  direction  of  O  ;  then 
falling  upon  the  glass,  it  is  again  refracted  in  the  direction  of 
P,  nearly  parallel  with  the  perpendicular  line  D. 

19.  In  all  cases  where  the  ray  passes  out  of  a  rarer  into  a 
denser  medium,  it  is  refracted  toward  a  perpendicular  line, 
raised  from  the  surface  of  the  denser  medium,  and  so,  when  it 

passes  out  of  a  denser,  into  a  rarer  medium,  it  is  refracted 

from  the  same  perpendicular. 

16.  What  is  a  medium  ?  In  what  direction  must  a  ray  of  light  pass  from  one  me- 
dium to  another,  to  be  refracted  1  17.  Will  a  ray  falling  perpendicularly  on  a  medium 
be  refracted  ?  18.  Explain  Fig.  172.  and  show  how  the  ray  E  is  refracted.  19.  When 
it  passes  out  of  a  rarer  into  a  denser  medium,  in  what  direction  is  it  refracted  ?  When 
it  passes  out  of  a  denser  into  a  rarer  medium,  in  what  direction  is  the  refraction  * 
Explain  this  by  Fig.  173. 


BEFRACTIQN    OF    LIGHT. 


223 


Let  the  medium  B,  Fig.  173,  be  glass,  and  the  medium  C, 
water.  The  ray  A,  as  it  falls  upon  the  medium  B,  is  refracted 
toward  the  perpendicular  line  E  D ;  but  when  it  enters  the  wa- 
ter, whose  refractive  power  is  less  than  that  of  glass,  it  is  not 
bent  so  near  the  perpendicular  as  before,  and  hence  it  is  re- 
fracted /rom,  instead  of  toward  the  perpendicular  line,  and  ap- 
proaches the  original  direction  of  the  ray  A  G,  when  passing 
through  the  air. 


FIG.  173. 


Water. 


Glass  and  Water. 


Air,  Water,  and  Glass. 


20.  The  cause  of  refraction  appears  to  be  the  power  of  at- 
traction, which  the  denser  medium  exerts  on  the  passing  ray; 
and  in  all  cases  the  attracting  force  acts  in  the  direction  of  a 
perpendicular  to  the  refracting  surface. 

21.  Refraction  by  Water. — The  refraction  of  the  rays  of 
light,  as  they  fall  upon  the  surface  of  the  water,  is  the  reason 
why  a  straight  rod,  with  one  end  in  the  water,  and  the  other 
end  rising  above  it,  appears  to  be  broken,  or  bent,  and  also  to 
be  shortened. 

Suppose  the  rod  A,  Fig.  174,  to  be  set  with  one  half  of  its 
length  below  the  surface  of  the  water,  and  the  other  half  above 
it.  The  eye  being  placed  in  an  oblique  direction,  will  see  the 
lower  end  apparently  at  the  point  0,  while  the  real  termination 
of  the  rod  would  be  at  N ;  the  refraction  will  therefore  make 
the  rod  appear  shorter  by  the  distance  from  O  to  N,  or  one- 
fourth  shorter  than  the  part  below  the  water  really  is.  The 
reason  why  the  rod  appears  distorted,  or  broken,  is,  that  we 


20.  What  is  the  cause  of  refraction  1  21.  What  is  the  reason  that  a  rod,  with  one 
end  in  the  water,  appears  distorted  and  shorter  than  it  really  is?  Why  does  the  wa- 
ter in  a  pond  appear  less  deep  than  it  really  is  7 


224  DOUBLE    REFRACTION. 

judge  of  the  direction  of  the  part  which  is  under  the  water,  by 
that  which  is  above  it,  and  the  refraction  of  the  rays  coining 
from  below  the  surface  of  the  water,  give  them  a  different  direc- 
tion, when  compared  with  those  coming  from  that  part  of  the 
rod  which  is  above  it.  Hence,  when  the  whole  rod  is  below 
the  water,  no  such  distorted  appearance  is  observed,  because 
then  all  me  rays  are  refracted  equally. 

For  the.reason  just  explained,  persons  are  often  deceived  in 
respect  to  the  depth  of  water,  the  refraction  making  it  appear 
much  more  shallow  than  it  really  is  ;  and  there  is  no  doubt  but 
the  most  serious  accidents  have  often  happened  to  those  who 
have  gone  into  the  water  under  such  deception ;  for  a  pond 
which  is  really  six  feet  deep,  will  appear  to  the  eye  only  a  little 
more  than  four  feet  deep. 


DOUBLE    REFRACTION. 


22.  By  double  refraction  is  meant  that  property  in  certain 
native  minerals,  by  which  they  transmit  two  images  of  a  single 
object. 

This  property  is  most  perfect  in  specimens  of  carbonate  of 
lime,  usually  called  Iceland  spar ;  the  latter  name  being  form- 
erly given  to  the  fine  specimens  from  that  country.  At  present, 
these'  rhomboids  are  found  in  most  primitive  limestone  countries. 

A  perfect  piece,  two  inches  in  diameter,  will  show  the  lines 
about  a  quarter  of  an  inch  apart,  the  greater  the  thickness  the 
more  distant  will  be  the  images  presented.  Sometimes  two  or 
three  pieces  of  different  sizes,  are  wanted  by  the  experimenters. 


If  a  piece  of  this  spar  be  laid  over  a  black  line,  and  then  be 
made  to  revolve  slowly,  it  will  be  observed  that  the  doubly  re- 
fractive power  increases  in  proportion  as  the  acute  angles  of  the 
rhomb  correspond  to  the  direction  of  the  line,  when  the  refrac- 
tion is  greatest,  or  the  two  lines  are  widest  apart.  On  the  con- 
trary, if  the  crystal  is  turned  in  either  direction  beyond  this 
point,  the  refracted  lines  approach  each  other,  until  the  short 
diagonal  or  obtuse  angles,  correspond  with  the  line,  when  the 
double  refraction  ceases  entirely,  and  only  a  single  line  appears. 

22.  What  is  meant  by  double  refraction  7    Explain  its  cause. 


REFLECTION    Of    LIGHT. 


225 


Explanation. — The  cause  of  this  difference  is,  that  when  the 
acute  angles  of  the  rhomb  correspond  to  the  black  line,  the  re- 
fracted ray  is  most  widely  separated  from  the  common  ray, 
which  depends  on  the  thickness  of  the  crystal,  but  when  the 
position  is  reversed,  the  common  ray  is  brought  into  the  exact 
line  of  the  refracted  one,  thus  forming  only  a  single  line. 


CATOPTRICS,    OR    THE    REFLECTION    OF    LIGHT. 

23.  If  a  boy  throws  his  ball  against  the  side  of  a  house  swiftly, 
and  in  a  perpendicular  direction,  it  will  bound  back  nearly  in 
the  line  in  which  it  was  thrown,  and  he  will  be  able  to  catch  it 
with  his  hands ;  but  if  the  ball  be  thrown  obliquely  to  the  right, 
or  left,  it  will  bound  away  from  the  side  of  the  house  in  the 
same  relative  direction  in  which  it  was  thrown. 

The  reflection  of  light,  so  far  as  regards  the  line  of  approach, 
and  the  line  of  leaving  a  reflecting  surface,  is  governed  by  the 
same  law. 

Thus,  if  a  sunbeam,  Fig.  175,  passing  through  a  small  aper- 
ture in  the  window-shutter  A,  be  permitted  to  fall  upon  the 
plane  mirror,  or  looking-glass,  C,  D,  at  right-angles,  it  will  be 
reflected  back  at  right-angles  with  the  mirror,  and  therefore 
will  pass  back  again  in  exactly  the  same  direction  in  which  it 
approached. 


FIG.  175. 


FIG.  176. 


FIG.  177. 


Reflection  of  Light. 

24.  But  if  the  ray  strikes  the  mirror  in  an  oblique  direction, 
it  will  also  be  thrown  off  in  an  oblique  direction,  opposite  to 
that  from  which  it  came. 

23.  Suppose  a  sunbeam  falls  upon  a  plane  mirror,  at  right  angles  with  its  surface, 
\.i  what  direction  will  it  be  reflected  ?  24.  Suppose  the  ray  falls  obliquely  on  its  sur- 
face in  what  direction  will  it  then  be  reflected  1  • 

10* 


226  MIRRORS. 

Let  a  ray  pass  toward  a  mirror  in  the  line  A  C,  Fig.  176,  it 
will  be  reflected  off  in  the  direction  C  D,  making  the  angles  1 
and  2  exactly  equal. 

The  ray  A  C,  is  called  the  incident  ray,  and  the  ray  C  D, 
the  reflected  ray ;  and  it  is  found,  in  all  cases,  that  "whatever 
angle  the  ray  of  incidence  makes  with  the  reflecting  surface,  or 
with  a  perpendicular  line  drawn  from  the  reflecting  surface,  ex- 
actly the  same  angle  is  made  by  the  reflected  ray. 

25.  From  these  facts,  arises  the  general  law  in  optics,  that 
the  angle  of  reflection  is  equal  to  the  angle  of  incidence. 

The  ray  A  C,  Fig.  177,  is  the  ray  of  incidence,  and  that 
from  G  to  D,  is  the  ray  of  reflection.  The  angles  which  A  C, 
make  with  the  perpendicular  line,  and  with  the  plane  of  the 
mirror,  are  exactly  equal  to  those  made  by  C  D,  with  the  same 
perpendicular,  and  the  same  plane  surface. 


26.  Mirrors  are  of  three  kinds,  namely,  plane,  convex,  and 
concave.     They  are  made  of  polished  metal,  or  of  glass  covered 
on  the  back  with  an  amalgam  of  tin  and  quicksilver. 

PLANE  MIRROR. — The  common  looking-glass  is  a  plane  mir- 
ror, and  consists  of  a  plate  of  ground  glass  so  highly  polished 
as  to  permit  the  rays  of  light  to  pass  through  it  with  little  in- 
terruption. On  the  back  of  this  plate  is  placed  the  reflecting 
surface,  which  consists  of  a  mixture  of  tin  and  mercury.  The 
glass  plate,  therefore,  only  answers  the  purpose  of  sustaining  the 
metallic  surface*  in  its  place, — of  admitting  the  rays  of  light  to 
and  from  it,  and  of  preventing  its  surface  from  tarnishing,  by 
excluding  the  air.  Could  the  metallic  surface,  however,  be  re- 
tained in  its  place,  and  not  exposed  to  the  air,  without  the  glass 
plate,  these  mirrors  would  be  much  more  perfect  than  they  are, 
since,  in  practice,  glass  can  not  be  made  so  perfect  as  to  trans- 
mit all  the  rays  of  light  which  fall  on  its  surface. 

27.  When  applied  to  the  plane  mirror,  the  angles  of  incidence 
and  of  reflection  are  equal,  as  already  stated ;  and  it  therefore 
follows,  that  when  the  rays  of  light  fall  upon  it  obliquely  in  one 
direction,  they  are  thrown  off  under  the  same  angle  in  the  op- 
posite direction. 

What  is  an  incident  ray  cf  light  1  What  is  a  reflected  ray  of  light?  25.  What  gen- 
eral law  in  optics  results  from  observations  on  the  incident  and  reflected  rays  1  26. 
How  many  kinds  of  mirrors  are  there  1  What  kind  of  mirror  is  the  common  look- 
ing-glass 7  Of  what  use  is  the  glass  plate  in  the  construction  ot  this  mirror  1  27.  Ex- 
plain Fig  178,  and  show  how  the  image  of  an  object  can  be  seen  in  a  plane  mirror 
when  the  real  object  is  invisible. 


MIRRORS. 


227 


Eyuai  Angles. 


This  is  the  reason  why  the  images  of  FIG-  I7a 

objects  can  be  seen  when  the  objects 
themselves  are  not  visible. 

Suppose  the  mirror,  A  B,  Fig.  1*78, 
to  be  placed  on  the  side  of  a  room,  and 
a  lamp  to  be  set  in  another  room,  but 
so  situated  as  that  its  light  would  shine 
upon  the  glass.  The  lamp  itself  could 
not  be  seen  by  the  eye  placed  at  E,  be- 
cause the  partition  D  is  between  them ; 
but  its  image  would  be  visible  at  E,  be-  B 
cause  the  angle  of  the  incident  ray, 
coming  from  the  light, 'and  that  of  the 
reflected  ray  which  reaches  the  eye,  are  equal. 

28.  An  image  from  a  plane  mirror  appears  to  be  just  as  far 
behind  the  mirror,  as  the  object  is  before  it,  so  that  when  a  per- 
son approaches  this  mirror,  his  image  seems  to  come  forward  to 
meet  him  ;  and  when  he  withdraws  from  it,  his  image  appears 
to  be  moving  backward  at  the  sam-e  rate. 

If,  for  instance,  one  end  of  a  rod,  two  feet  long,  ^e  made  to 
touch  the  surface  of  such  a  mirror,  this  end  of  the  rod,  and  its 
image,  will  seem  nearly  to  touch  each  other,  there  being  only 
the  thickness  of  the  glass  between  them ;  while  the  other  end 
of  the  rod,  and  the  other  end  of  its  image,  will  appear  to  be 
equally  distant  from  the  point  of  contact. 

29.  The  reason  of  this  is  ex- 
plained on  the  principle  that  the 
angle  of  incidence  and  that  of 
reflection  are  equal. 

Suppose  the  arrow  A  to  be 
the  object  reflected  by  the  mir- 
ror D  C,  Fig.  179  ;  the  incident 
rays  A,  flowing  from  the  end  of 
the  arrow,  being  thrown  back 
by  reflection,  will  meet  the  eye 
in  the  same  state  of  divergence 
that  they  would  do,  if  they  pro- 
ceed to  the  same  distance  be- 
hind the  mirror,  that  the  eye  is 

before  it,  as  at  0.     Therefore,  by  the  same  law,  the  reflected 
rays,  where  they  meet  the  eye  at  E,  appear  to  diverge  from  a 


FIG.  179. 


Plane  Mirror. 


28.  The  image  of  an  object  appears  just  as  far  behind  a  plane  mirror, 
Is  before  it.    29.  Explain  Fig.  1/9,  and  show  why  this  is  the  case. 


the  object 


228  MIRRORS. 

point  H,  just  as  far  behind  the  mirror  as  A  is  before  it,  and 
consequently  the  end  of  the  arrow  most  remote  from  the  glass 
will  appear  to  be  at  H,  or  the  point  where  the  approaching 
rays  would  meet,  were  they  continued  onward  behind  the  glass. 
The  rays  flowing  from  every  other  part  of  the  arrow  follow  the 
same  law ;  and  thus  every  part  of  the  image  seems  to  be  at 
the  same  distance  behind  the  mirror  that  the  object  really  is 
before  it. 

30.  In  a  plane  mirror,  a  person  may  see  his  whole  image, 
when  the  mirror  is  only  half  as  long  as  himself,  let  him  stand 
at  any  distance  from  it  whatever. 

This  is  also  explained  by  the  law,  that  the  angles  of  incidence 
and  reflection  are  equal.  If  the  mirror  be  elevated  so  that  the 
ray  of  light  from  the  eye  falls  perpendicularly  upon  the  mirror, 
this  ray  will  be  thrown  back  by  reflection  in  the  same  direction, 
so  that  the  incident  and  reflected  ray  by  which  the  image  of 
the  eyes  and  face  are  formed,  will  be  nearly  parallel,  while  the 
ray  flawing  from  his  feet  will  fall  on  the  mirror  obliquely,  and 
will  be  rejected  as  obliquely  in  the  contrary  direction,  and  so 
of  all  the  other  rays  by  which  the  image  of  the  different  parts 
of  the  person  is  formed. 

This  will  be  under- 
stood by  Fig.  180, 
where  the  ray  of  light 
A  B,  proceeding  from 
the  eye,  falling  perpen- 
dicularly on  the  plane 
mirror  IB  D,  will  be  re- 
flected back  in  the  same 

line;    but  the  ray  C  D  Mirror  half  the  Length  of  the  Object, 

coming  from  the  feet, 

which  falls  obliquely  on  the  mirror,  will  be  reflected  back  under 
the  same  angle  in  the  line  D  A ;  and  since  we  see  objects  in 
the  direction  of  the  reflected  rays,  and  the  image  appears  at  the 
same  distance  behind  the  mirror  that  the  object  is  before  it, 
(28,)  we  must  continue  the  line  A  D  to  the  feet,  E,  and  for  the 
same  reason,  the  rays  A  B,  from  the  eye,  must  be  prolonged  to 
F,  as  far  behind  the  mirror  as  the  line  E  extends,  where  the 
whole  image  will  be  represented. 

Now,  the  line  D  E,  behind  the  mirror,  is  just  equal  to  D  A 


30.  What  must  be  the  comparative  length  of  a  plane  mirror  in  which  a  person  may 
see  his  whole  image  1  Explain,  by  the  lines  in  Fig.  180,  why  it  is  that  a  lady  may  see 
herself  in  a  mirror  one  half  her  length. 


MIRRORS. 


229 


FIG.  181. 


Convex  Mirror. 


before  it ;  and  the  distance  of  A  C  is  just  twice  that  of  B  D ; 
therefore,  the  whole  person  is  seen  in  a  mirror  of  half  its  length, 
the  image  being  as  far  behind  the  reflector  as  the  object  is 
before  it. 

31.  A  shorter  mirror  would  not  show  the  whole  person,  be- 
cause the  rays  coming  from  the  feet  would  fall  so  obliquely  upon 
it  as  to  be  reflected  above  his  head,  and  thus  could  not  be  seen ; 
but  another  placed  there  might  see  the  whole  image,  though 
the  owner  could  not. 

32.  CONVEX  MIRRO'R. — A  con- 
vex mirror  is  a  part  of  a  sphere, 
or  globe,  reflecting  from  the  out- 
side. 

Suppose  Fig.  181  to  be  a 
sphere,  then  the  part  from  A  to  O, 
would  be  a  section  of  the  sphere. 
Any  part  of  a  hollow  ball  of  glass, 
with  an  amalgam  of  tin  and  quick- 
silver spread  on  the  inside,  or  any 
part  of  a  metallic  globe  polished 
on  the  outside,  would  form  a  con- 
vex mirror. 

The  axis  of  a  convex  mirror,  is  a  line,  as  C  B,  passing  through 
its  center. 

33.  Divergent  and  Convergent  FIG.  182. 
Rays. — Rays  of  light  are  said  to 

diverge,  when  they  proceed  from 
the  same  point,  and  constantly  re- 
cede from  each  other,  as  from  the 
point  A,  Fig.  182.  Rays  of  light 
are  said  to  converge,  when  they 
approach  in  such  a  direction  as 
finally  to  meet  at  a  point,  as  at  B,  Fig.  182. 

The  image  formed  by  a  plane  mirror,  as  we  have  already 
seen,  is  of  the  same  size  as  the  object,  but  the  image  reflected 
from  the  convex  mirror  is  always  smaller  than  the  object. 

The  law  which  governs  the  passage  of  light  with  respect  to 
the  angles  of  incidence  and  reflection,  to  and  from  the  convex 
mirror,  is  the  same  as  already  stated,  for  the  plane  mirror. 

34.  From  the  surface  of  a  plane  mirror,  parallel  rays  are  re- 
si.  Why  can  not  a  person  see  his  whole  figure  in  a  mirror  less  than  half  his  length? 

32.  What  is  a  convex  mirror  1  What  is  the  axis  of  a  convex  mirror  7  33.  What  are 
diverging  rays  1  What  are  converging  rays!  What  law  governs  the  passage  of  light 
from  and  to  the  convex  mirror  7 


Rays  of  Light. 


230 


MIRRORS. 


fleeted  parallel ;  but  the  convex  mirror  causes  parallel  rays  fall- 
ing on  its  surface  to  diverge,  by  reflection. 

To  make  this  understood,  let  1, 
2,  3,  Fig.  183,  be  parallel  rays,  FIG.  183. 

falling  on  the  surface  of  the  convex 
reflector,  of  which  A  would  be  the 
center,  were  the  reflector  a  whole 
sphere.  The  ray  2  is  perpendicu- 
lar to  the  surface  of  the  mirror,  for 
when  continued  in  the  same  direc- 
tion, it  strikes  the  axis,  or  center 
of  the  circle  A.  The  two  rays,  1 
and  3,  being  parallel  to  this,  all 
three  would  fall  on  a  plane  mirror 
in  a  perpendicular  direction,  and 
consequently  would  be  reflected  in 
the  lines  of  their  incidence.  But 
the  obliquity  of  the  convex  surface,  Divergent  Rays. 

it  is  obvious,  will  render  the  direc- 
tion of  the  rays  1  and  3  oblique  to  that  surface,  for  the  same 
reason  that  2  is  perpendicular  to  that  part  of  the  circle  on  which 
it  falls.  Rays  falling  on  any  part  of  this  mirror,  in  a  direction 
which,  if  continued  through  the  circumference,  would  strike  the 
center,  are  perpendicular  to  the  side  where  they  fall.  Thus,  the 
dotted  lines,  C  A  and  D  A,  are  perpendicular  to  the  surface,  as 
well  as  2. 

Now  the  reflection  of  the  ray  2,  will  be  back  in  the  line  of 
its  incidence,  but  the  rays  1  and  3,  falling  obliquely,  are  reflected 
under  the  same  angles  as  those  at  which  they  fell,  and  there- 
fore their  lines  of  reflection  will  be  as  far  without  the  perpen- 
dicular lines  C  A  and  D  A,  as  the  lines  of  their  incident  rays, 
1  and  3,  are  within  them,  and  consequently  they  will  diverge 
in  the  direction  of  E  and  O  ;  and  since  we  always  see  the  image 
in  the  direction  of  the  reflected  ray,  an  object  placed  at  one,  would 
appear  behind  the  surface  of  the  mirror,  at  N,  or  in  the  direc- 
tion of  the  line  O  N. 

35.  Plane  Surfaces. — Perhaps  the  subject  of  the  convex 
mirror  will  be  better  understood,  by  considering  its  surface  to 
be  formed  of  a  number  of  plane  faces,  indefinitely  small.  In 
this  case,  each  point  from  which  a  ray  is  reflected,  would  act  in 


34.  Are  parallel  rays  falling  on  a  convex  mirror,  reflected  parallel?  Explain  Fig. 
183.  35.  How  is  the  action  of  the  convex  mirror  illustrated  by  a  number  of  piano 
mirrors  ?  Explain  Figs.  184  and  185. 


MIRRORS.  231 

the  same  manner  as  a  plane  mirror,  and  the  whole,  in  the  man- 
ner of  a  number  of  minute  mirrors  inclined  from  each  other. 

Suppose  A  and  B,  Fig.  184,  to  be  the  points  on  a  convex 
mirror,  from  which  the  two  parallel  rays,  C  and  D,  are  reflected. 
Now,  from  the  surface  of  a  plane  mirror,  the  reflected  rays 
would  be  parallel,  whenever  the  incident  ones  are  so,  because  each 
will  fall  upon  the  surface  under  the  same  angles.  But  it  is  ob- 
vious, in  the  present  case,  that  these  rays  fall  upon  the  surfaces, 
A  and  B,  under  different  angles,  as  respects  the  surfaces,  C  ap- 
proaching in  a  more  oblique  direction  than  D ;  consequently  C 
is  reflected  more  obliquely  than  D,  and  the  two  reflected  rays, 
instead  of  being  parallel  as  before,  diverge  in  the  direction  of  N 
andO.  * 

FIG.  184.  FIG.  185. 


Again,  the  two  converging  rays  A  and  B,  Fig.  185,  without 
the  interposition  of  the  reflecting  surfaces,  would  meet  at  C,  but 
because  the  angles  of  reflection  are  equal  to  those  of  incidence, 
and  because  the  surfaces  of  reflection  are  inclined  from  each 
other,  these  rays  are  reflected  less  convergent,  and  instead  of 
meeting  at  the  same  distance  before  the  mirror  that  C  is  behind 
it,  are  sent  off  in  the  direction  of  D,  at  which  point  they  meet. 

36.  "  Thus  parallel  rays  falling  on  a  convex  mirror,  are  ren- 
dered divergent  by  reflection  ;  converging  rays  are  made  less 
convergent,  or  parallel,  and  diverging  rays  more  divergent. 

The  effect  of  the  convex  mirror,  therefore,  is  to  disperse  the 

36.  What  effect  does  the  convex  mirror  have  upon  parallel  rays  by  reflection  ? 
What  is  its  effect  en  con  verging  rays  1  What  is  its  effect  on  diverffing  rays?  Do  the 
rays  oflight  proceed  only  from  the  extremities  of  objects,  as  represented  in  figures, 
or  from  all  their  parts  ?  Do  all  the  rays  of  light  proceeding  from  an  object  enter  the 
eye,  or  only  a  few  of  them  1 


232  MIRRORS. 

rays  of  light  in  all  directions ;  and  it  is  proper  here  to  remind 
the  pupil,  that  although  the  rays  of  light  are  represented  on 
paper  by  single  lines,  there  are,  in  fact,  probably  millions  of 
rays,  proceeding  from  every  point  of  all  visible  bodies.  Only  a 
comparatively  small  number  of  these  rays,  it  is  true,  can  enter 
the  eye,  for  it  is  only  by  those  which  proceed  in  straight  lines 
from  the  different  parts  of  the  object,  and  enter  the  pupil,  that 
the  sense  of  vision  is  excited. 

37.  When,  therefore,  it  is  said,  that  the  convex  mirror  dis- 
perses the  rays  of  light  which  fall  upon  it  from  any  object,  and 
when  the  direction  of  these  reflected  rays  are  shown  only  by 
single  lines,  it  must  be  remembered,  that  each  line  represent* 
pencils  of  rays,  and  that  the  ligh*  not  only  flows  from  the  parts 
of  the  object  thus  designated,  but  from  all  the  other  parts. 
Were  this  not  the  case,  the  object  would  be  visible  only  at  cer- 
tain points. 

38.  CURVED  IMAGES. — The  images  of  objects  reflected  from 
the  convex  mirror,  appear  curved,  because  their  different  parts 
are  not  equally  distant  from  its  surface. 

If  the  object  A  be  placed 

obliquely  before  the  convex  FIG<  186- 

mirror,  Fig.  186,  then  the 
converging  rays  from  its  two 
extremities  falling  obliquely 
on  its  surface,  would,  were 
they  prolonged  through  the 
mirror,  meet  at  the  point  C, 
behind  it.  But  instead  of 
being  thus  continued,  they  Curved  image. 

are  thrown  back  by  the  mir- 
ror in  less  convergent  lines,  which  meet  the  eye  at  E,  it  being, 
as  we  have  seen,  one  of  the  properties  of  this  mirror,  to  reflect 
converging  rays  less  convergent  than  before. 

The  image  being  always  seen  in  the  direction  from  which  the 
rays  approach  the  eye,  it  appears  behind  the  mirror  at  D.  If 
the  eye  be  kept  in  the  same  position,  and  the  object,  A,  be 
moved  further  from  the  mirror,  its  image  will  appear  smaller, 
in  a  proportion  inversely  to  the  distance  to  which  it  is  removed. 
Consequently,  by  the  same  law,  the  two  ends  of  a  straight  ob- 
ject will  appear  smaller  than  its  middle,  because  they  are  further 

37.  What  would  be  the  consequence,  if  the  rays  of  light  proceeded  only  from  the 
parts  of  an  object  shown  in  diagrams?  38.  Why  do  the  images  of  objects  reflected 
from  convex  mirrors  appear  curved?  Why  do  the  features  of  the  face  appear  out 
of  proportion  by  this  mirror? 


MIRRORS. 


233 


from  the  reflecting  surface  of  the  mirror.  Thus,  the  images  of 
straight  objects,  held  before  a  convex  mirror,  appear  curved,  and 
for  the  same  reason,  the  features  of  the  face  appear  out  of  pro- 
portion, the  nose  being  too  large,  and  the  cheeks  too  small,  or 
narrow.  • 

39.  Why  Objects  appear  Large  or  Small. — Objects  appear  to 
us  large  or  small,  in  proportion  to  the  angle  which  the  "rays  of 
light,  proceeding  from  their  extreme  parts,  form,  when  they 
meet  at  the  eye.  For  it  is  plain  that  the  half  of  any  object  will 
appear  under  a  less  angle  than  the  whole,  and  the  quarter 
under  a  less  angle  still.  Therefore  the  smaller  an  object  is,  the 
smaller  will  be  the  angle  under  which  it  will  appear  at  a  given 
distance.  Hence  the  image  of  an  object,  when  reflected  from  the 
convex  mirror,  appears  smaller  than  the  object  itself.  This  will 
be  understood  by  Fig.  1&7. 

Suppose  the  rays  flowing  from  the  extremities  of  the  object 
A,  to  be  reflected  back  to  C,  under  the  same  degrees  of  con- 
vergence at  which  they  strike  the  mirror;  then,  as  in  the  plane 
mirror,  the  image  D  would  appear  of  the  same'size  as  the  ob- 
ject A;  for  if  the  rays  from  A  were  prolonged  behind  the 
mirror,  they  would  meet  at  B,  but  forming  the  same  angle,  by 
reflection,  that  they  would  do,  if  thus  prolonged,  the  object  seen 
from  B,  and  its  image  from  C,  would  appear  of  the  same 
dimensions. 


FIG.  187. 


FIG.  188. 


Object  Diminished. 


Convex  Mirror. 


But  instead  of  this,  the  rays  from  the  arrow  A,  being  rendered 
less  convergent  by  reflection,  are  continued  onward,  and  meet 
the  eye  under  a  more  acute  angle  than  at  C,  the  angle  under 
which  they  actually  meet,  being  represented  at  E,  consequently 

39.  Why  does  an  image  reflected  from  a  convex  surface  appear  smaller  than  the 
object  1    Why  does  the  half  of  an  object  appear  to  the  eye  smaller  than  the  whole  ? 


234  MIRRORS. 

the  image  of  the  object  is  shortened  in  proportion  to  the  acute- 
ness  of  this  angle,  and  the  object  appears  diminished  as  repre- 
sented at  O. 

40.  The  image  of  an  object  appears  less,  as  the  object  is  re- 
moved to  a  greater  distance  from  a  convex  mirror. 

To  explain  this,  let  us  suppose  that  the  arrow  A,  Fig.  188, 
is  diminished  by  reflection  from  the  convex  surface,  so  that  its 
image  appearing  at  D,  with  the  eye  at  C,  shall  seem  as  much 
smaller  in  proportion  to  the  object,  as  D  is  less  than  A.  Now, 
keeping  the  eye  at  the  same  distance  from  the  mirror,  withdraw 
the  object,  so  that  it  shall  be  equally  distant  with  the  eye,  and 
the  image  will  gradually  diminish,  as  the  arrow  is  removed. 

The  reason  will  be  made  plain  by  the  next  figure  ;  for  as  the 
arrow  is  moved  backward,  the  angle  at  C,  Fig.  189,  must  be 
diminished,  because  the  rays  flowing  from  the  extremities  of  the 
object  fall  a  greater  distance  before  they  reach  the  surface  of 
the  mirror ;  and  as  the  angles  of  the  reflected  rays  bear  a  pro- 
portion to  those  of  the  incident  ones,  so  the  angle  of  vision  will 
become  less  in  proportion,  as  the  object  is  withdrawn.  The 
effect,  therefore,  of  withdrawing  the  object,  is  first  to  lessen  the 
distance  between  the  converging  rays,  flowing  from  it,  at  the 
point  where  they  strike  the  mirror,  and  as  a  consequence,  to 
diminish  the  angle  under  which  the  reflected  rays  convey  its 
image  to  the  eye. 

41.  Why  the  Image  seems  near  the  Surface. — In  the  plane 
mirror,  as  already  shown,  the  image  appears  exactly  as  far  be- 
hind the  mirror  as  the  object  is  before  it,  but  the  convex  mirror 
shows  the  image  just  under  the  surface,  or,  when  the  object  is 
removed  to  a  distance,,  a  little  way  behind  it.     To  understand 
the  reason  of  this  difference,  it  must  be  remembered,  that  the 
plane  mirror  makes  the  image  seem  as  far  behind,  as  the  object 
is  before  it,  because  the  rays  are  reflected  in  the  same  relative 
position  at  which  they  fall  upon  its  surface.     Thus  parallel  rays 
are  reflected  parallel;    divergent  rays  equally  divergent,  and 
convergent  rays  equally  convergent.     But  the  convex  mirror,  as 
also  above  shown,  (36,)  reflects  convergent  rays  less  convergent, 
and  divergent  rays  more  divergent,  and  it  is  from  this  property 
of  the  convex  mirror  that  the  image  appears  near  its  surface, 


40.  jflfow  is  the  image  affected  when  the  object  is  withdrawn  from  the  surface  of  a 
convex  mirror  1  Explain  Figs.  187  and  188,  and  show  the  reason  why  the  images  are 
diminished  when  the  objects  are  removed  from  the  convex  mirror.  What  is  said  to 
be  the  effect  of  withdrawing  the  object  from  a  convex  surface,  and  what  the  conse- 
quence on  the  angle  of  reflected  rays  ?  41.  Explain  the  reason  why  the  image  ap- 
pears near  the  surface  of  the  convex  mirro*. 


MIRRORS.  235 

FIG.  189. 


''I-'—  0 


and  not  as  far  behind  it  as  the  object  is  before  it,  as  in  the 
plane  mirror. 

Let  us  suppose  that  A,  Fig.  190,  is  a  luminous  point,  from 
which  a  pencil  of  diverging  rays  falls  upon  a  convex  mirror. 
These  rays,  as  already  demonstrated,  will  be  reflected  more 
divergent,  and  consequently  will  meet  the  eye  at  E,  in  a  wider 
state  of  dispersion  than  they  fell  upon  the  mirror  at  O.  Now, 
as  the  image  will  appear  at  the  point  where  the  diverging  rays 
would  converge  to  a  focus  in  a  contrary  direction,  were  they 
prolonged  behind  the  mirror,  so  it  can  not  appear  as  far  behind 
the  reflecting  surface  as  the  object  is  before  it,  for  the  more 
widely  the  rays  meeting  at  the  eye  are  separated,  the  shorter 
will  be  the  distance  at  which  they  will  come  to  a  point.  The 
image  will,  therefore,  appear  at  N,  instead  of  appearing  at  an 
equal  distance  behind  the  mirror  that  the  object  A  is  before  it. 


CONCAVE    MIRROR. 


42.  The  reflection  of  the  concave  mirror  takes  "place  from  its 
inside,  or  concave  surface,  while  that  of  the  convex  mirror  is 
from  the  outside,  or  convex  surface.  Thus  the  section  of  a 
metallic  sphere,  polished  on  both  sides,  is  both  a  concave  and 
convex  mirror,  as  one  or  the  other  side  in  employed  for  reflection. 

The  effects  and  phenomena  of  this  mirror  will  therefore  be,  in 
many  respects,  directly  the  contrary  from  those  already  detailed 
in  reference  to  the  convex  mirror. 

From  the  plane  mirror,  the  relation  of  the  incident  rays  is 
not  changed  by  reflection ;  from  the  convex  mirror  they  are  dis- 

42.  What  is  the  shape  of  the  concave  mirror,  and  in  what  respect  dees  it  differ 
from  the  convex  mirror  ?  How  may  convex  and  concave  mirrors  be  united  in  the 
tame  instrument?  What  is  the  difference  of  effect  between  the  concave,  convex 
ind  plane  mirrers,  on  the  reflected  rays  1 


236  MIRRORS. 

persed ;  but  the  concave  mirror  renders  the  rays  reflected  from 
it  more  convergent,  and  tends  to  concentrate  them  into  a  focus. 

The  surface  of  the  concave  mirror,  like  that  of  the  convex, 
may  be  considered  as  a  great  number  of  minute  plane  mirrors, 
inclined  to,  instead  of  from,  each  other  at  certain  angles,  in  pro- 
portion to  its  concavity. 

The  laws  of  incidence  and  reflection  are  the  same,  when  ap- 
plied to  the  concave  mirror  as  those  already  explained  in  refer- 
ence to  the  other  mirrors. 

43.  Plane  Mirrors  Inclined.— In  refer-  PIG>  191- 
ence  to  the  concave  mirror,  let  us,  in  the 

first  place,  examine  the  effect  of  two  plane 
mirrors  inclined  to  each  other,  as  in  Fig. 
191,  on  parallel  rays  of  light.  The  inci- 
dent rays,  A  and  B,  being  parallel  before 
they  reach  the  reflectors,  are  thrown  off  .at 
unequal  angles  in  respect  to  each  other,  for 
B  falls  on  the  mirror  more  obliquely  than 
A,  and  consequently  is  thrown  off  more  ob-  /  / 

liquely  in  a  contrary  direction,  therefore,  the 
angles  of  reflection  being  equal  to  those  of  incidence,  the  two 
rays  meet  at  C. 

Thus  we  see  that  the  effect  of  two  plane  mirrors  inclined  to 
each  other,  is  to  make  parallel  rays  converge  and  meet  in  a 
focus. 

The  effect  of  this  mirror,  as  we.  have  seen,  being  to  render 
parallel  rays  convergent,  the  same  principle  will  render  diverg- 
ing rays  parallel,  and  converging  rays  still  more  convergent. 

44.  Focus  of  a  Concave  Mirror. — The  focus  of  a  concave 
mirror  is  the*  point  where  the  rays  are  brought  together  by  re- 
flection.    The  center  of  concavity  in  a  concave  mirror,  is  the 
center  of  the  sphere,  of  which  the  mirror  is  a  part.     In  all  con- 
cave mirrors,  the  focus  of  parallel  rays,  or  rays  falling  directly 
from  the  sun,  is  at  the  distance  of  half  the  semi-diameter  of  the 
sphere,  or  globe,  of  which  the  reflector  is  a  part. 

Thus,  the  parallel  rays  1,  2,  3,  <fcc.,  Fig.  192,  all  meet  at  the 
point  O,  which  is  half  the  distance  between  the  center  A,  of  the 
whole  sphere,  and  the  surface  of  the  reflector,  and  therefore  one 
quarter  the  diameter  of  the  whole  sphere,  of  which  the  mirror 
is  a  part. 

43.  In  what  respect  may  the  concave  mirror  be  considered  as  a  number  of  plane 
mirrors?  44.  What  is  the  focus  of  a  concave  mirror!  At  what  distance  from  its 
surface  is  the  focus  of  parallel  rays  in  this  mirror? 


MIRRORS. 


237 


45.  Principal  Focus. 
— In  concave  mirrors,  of 
all  dimensions,  the  re- 
flected rays  follow  the 
same  law ;  that  is,  par- 
allel rays  meet  and  cross 
each  other  at  the  dis- 
tance of  one-fourth  the 
diameter  of  the  sphere 
of  which  they  are  sec- 
tions. This  point  is  call- 
ed the  principal  focus 
of  the  reflector. 

But  if  the   incident 
rays  are  divergent,  the 
focus   will  be  removed 
to  a  greater  distance  from  the  surface  of  the  mirror,  than  when  • 
they  are  parallel,  in  proportion  to  their  divergence. 


FIG.  193. 


FIG.  194 


Divergent  Rays. 


Convergent  Rays. 


This  might  be  inferred  from  the  general  laws  of  incidence  and 
reflection,  but  will  be  made  obvious  by  Fig.  193,  where  the 
diverging  rays  1,  2,  3,  4,  form  a  focus  at  the  point  O,  whereas, 
had  they  been  parallel,  their  focus  would  have  been  at  A.  That 
is,  the  actual  focus  is  at  the  center  of  the  sphere,  instead  of  be- 
ing half  way  between  the  center  and  circumference,  as  is  the 
case  when  the  incident  rays  are  parallel.  The  real  focus,  there- 
fore, is  beyond,  or  without,  the  principal  focus  of  the  mirror. 


45.  What  is  the  principal  focus  of  a  concave  mirror?  If  the  incident  rays  are  di 
vergent.  where  will  be  the  focus  ?  If  the  incident  rays  are  convergent,  where  will  be 
the  focus  7 


238 


MIRRORS. 


By  the  same  law,  converging  rays  will  form  a  point  within 
the  principal  focus  of  the  mirror. 

Thus,  were  the  fays  falling  on  the  mirror,  Fig.  194,  parallel, 
the  focus  would  be  at  A ;  but  in  consequence  of  their  previous 
convergence,  they  are  brought  together  at  a  less  distance  than 
the  principal  focus,  and  meet  at  O. 

46.  OBJECTS  WITHIN  THE  Focus. — 'The  concave  mirror,  when 
the  object  is  nearer  to  it  than  the  principal  focus,  presents  the 
image  larger  than  the  object,  erect,  and  behind  the  mirror. 

To  explain  this,  let  us 

suppose    the    object   A,  FIG.  195. 

Fig.  195,  to  be  placed 
before  the  mirror,  and 
nearer  to  it  than  the  prin- 
cipal focus.  Then  the 
rays  proceeding  from  the 
extremities  of  the  object 
without  interruption, 
would  continue  to  diverge 
in  the  lines  O  and  N,  as 
seen  behind  the  mirror ; 
but,  by  reflection  they 
are  made  to  diverge  less 
than  before,  and  conse- 
quently to  make  the  an- 
gle under  which  they  meet  more  obtuse  at  the  eye  B,  than  it 
would  be  if  they  continued  onward  to  E,  where  they  would 
have  met  without  reflec- 
tion. The  result,  there- 
fore, is  to  render  the  image 
H,  upon  the  eye,  as  much 
larger  than  the  object  A, 
as  the  angle  at  the  eye 
is  more  obtuse  than  the 
angle  at  C. 

47.  Magnified  Human 
Face. — A  more  striking 
illustration  of  this  princi- 
ple is  seen  at  Fig.  196. 
When  the  concave  mir- 


Object  within  the  Focus. 


Magnified  Human  Face. 


46.  When  will  the  image  from  a  concave  mirror  oe  larger  than  the  object,  erect, 
and  behind  the  mirror  1  Explain  Fig.  195,  and  show  why  the  image  is  larger  than 
the  object.  47.  What  is  the  effect  when  the  face  is  seen  within  the  principal  focus  of 
this  mirror  1 


MIRRORS. 


239 


ror  is  large,  say  six  inches  in  diameter  and  eight  or  ten  inches 
focal  distance,  it  exhibits  the  human  face  of  enormous  bulk,  the 
spectator  being  frightened  at  the  size  and  coarseness  of  his  own 
features.  Thus,  if  the  face  be  presented  within  the  principal 
focus  of  the  mirror,  as  at  B,  the  magnified  image  will  be  seen 
as  far  behind  the  mirror  as  the  face  is  before  it,  as  at  A,  and 
will  appear  two  or  three  times  the  size  of  the  face,  according*  to 
the  power  of  the  reflector;  the  reason  of  which  has  already 
been  explained  and  illustrated  by  Fig.  196. 

48.  Curious  Deceptions  by    Concave   Mirrors. — From    the 
property  of  the  concave  mirror  to  form  an  inverted  image  of 
the  object  suspended  in  the  air,  many  curious  and  surprising 
deceptions  may  be  produced.     Thusy  when  the  mirror,  the  ob- 
ject, and  the  light,  are  placed  so  that  they  can  not  be  seen, 
(which  may  be  done  by  placing  a  screen  before  the  light,  and 
permitting  the  reflected  rays  to  pass  through  a  small  aperture 
in  another  screen,)  the  person  mistakes  the  image  of  the  object 
for  its  reality,  and  not  understanding  the  deception,  thinks  he 
sees  persons  walking  with  heads  downward,  and  cups  of  water 
turned  bottom  upward,  without  spilling  a  drop.     Again,   he 
sees  clusters  of  delicious  fruit,  and  when  invited  to  help  himself, 
on  reaching  out  his  hand  for  that  purpose,  he  finds  that  the  ob- 
ject either  suddenly  vanishes  from  his  sight,  owing  to  bis  having 
moved  his  eye  out  of  the  proper  range,  or  that  it  is  intangible. 

49.  One  method  of  ef- 
fecting such  deceptions,  is 
shown  by  Fig.  197,  where 
A  is  a  large  concave  mir- 
ror, six  or  eight  inches  in 
diameter,  placed   on   the 
back  part  of  a  dark  box ; 
the  performer  D,  is  con- 
cealed from  the  spectators 
by  the  partition    C;  the 
strong  light  E,  is  also  con- 
cealed by  the  partition  I, 
but  is  thrown  upon  the 
actor,   or   any   thing    he 
holds  in  his  hand.     If  he 
holds  a  book,  as  shown  by 


FIG.  197. 


Deceptions  by  Concave  Mirrors. 


48.  What  property  has  the  concave  mirror,  by  which  singular  deceptions  may  be 
produced?  What  are  these  deceptions?  49.  Explain  Fig.  197,  and  show  how  th» 
deception  is  produced. 


240  MIRRORS. 

the  figure,  the  light  reflected  from  A,  will  pass  between  the  par- 
titions, G  and  I,  to  the  mirror,  and  will  reflect  the  image  of  the 
book  to  Z,  where  it  will  appear  so  distinct  and  tangible,  that  a 
person  looking  through  the  opening  at  X,  will  have  no  doubt 
that  it  is  a  real  book,  and  will  be  much  astonished  to  find,  when 
be  puts  out  his  hand  to  take  it,  that  it  -has  no  substance,  and 
that  his  hand  will  pass  through  it,  as  though  it  was  nothing 
but  a  shadow,  which  he  can  not  at  first  be  made  to  believe  is 
the  case. 

50.  Heat  Produced  by  a    Concave  Mirror. — The  concave 
mirror  having  the  property  of  converging  the  rays  of  light,  is 
equally  efficient  in  concentrating  the  rays  of  heat,  either  separ- 
ately or  with  the  light.     When,  therefore,  such  a  mirror  is  pre- 
sented to  the  rays  of  the  sun,  it  brings  them  to  a  focus,  so  as 
to  produce  degrees  of  heat  in  proportion, to  the  extent  and  per- 
fection of  its  reflecting  surface.     A  metallic  mirror  of  this  kind, 
of  only  four  or  six  inches  in  diameter,  will  fuse  metals,  set  wood 
on  fire,  &c. 

51.  Experiment  with  a  Hot  Ball. — As  the  parallel  rays  of 
heat  or  light  are  brought  to  a  focus  at  the  distance  of  one  quar- 
ter of  the  diameter  of  the  sphere,  of  which  the  reflector  is  a 
section,  so  if  a  luminous  or  heated  body  be  placed  at  this  point, 
the  rays  from  such  body  passing  to  the  mirror  will  be  reflected 
from  all  parts  of  its  surface,  in  parallel  lines ;  and  the  rays  so 
reflected  by  the  same  law,  will  be  brought  to  a  focus  by  an- 
other mirror  standing  opposite  to  this. 

52.  Suppose  a  red-hot  ball  to  be  placed  in  the  principal  focus 
of  the  mirror  A,  Fig.  198,  the  rays  of  heat  and  light  proceed- 
ing from  it  will  be  reflected  in  the  parallel  lines  1,  2,  3,  &c. 
The  reason  of  this  is  the  same  as  that  .which  causes  parallel 
rays,  when  falling  on  the  mirror,  to  be  converged  to  a  focus. 
The  angles  of  incidence  being  equal  to  those  of  reflection,  it 
makes  no  difference  in  this  respect,  whether  the  rays  pass  to  or 
from  the  focus.     In  one  case,  parallel  incident  rays  from  the 
sun,  are  concentrated  by  reflection ;  and  in  the  other,  incident 
diverging  rays,  from  the  heated  ball,  are  made  parallel  by 
reflection. 

The  rays,  therefore,  flowing  from  the  hot  ball  to  the  mirror 


50.  Will  the  concave  mirror  concentrate  the  rays  of  heat,  as  well  as  those  of  light  ? 
51.  Suppose  a  luminous  body  be  placed  in  the  focus  of  a  concave  mirror,  in  what  di- 
rection will  its  rays  be  reflected]  52.  Explain  Fig  198,  and  show  why  the  rays 
from  the  focus  of  A  are  concentrated  in  the  focus  B.  What  curious  experiments  may 
be  made  by  two  concave  mirrors  placed  opposite  to  each  other  by  a  hot  ball?  How 
may  it  be  shown  that  heat  and  light  are  distinct  principles  ? 


MIRRORS. 

FIG.  198. 

± 


241 


Reflection  of  Heat. 

A,  are  thrown  into  parallel  lines  by  reflection,  and  these  re- 
flected rays,  in  respect  to  the  mirror  B,  become  the  rays  of  inci- 
dence, which  are  again  brought  to  a  focus  by  reflection. 

Thus  the  heat  of  the  ball,  by  being  placed  in  the  focus  of  one 
mirror,  is  brought  to  a  focus  by  the  reflection  of  the  other 
mirror. 

To  show  that  heat  and  light  are  separate  principles,  place  a 
piece  of  phosphorus  in  the  focus  of  B,  and  when  the  ball  is  so 
cool  as  not  to  be  luminous,  remove  the  screen,  and  the  phos- 
phorus will  instantly  inflame. 

53.  Deception  by  Several  Mirrors. — The  wonderful  feat  of 
reading  through  a  brick,  sometimes  exhibited  in  the  streets,  at 
a  penny  a  head,  is  explained  by  Fig.  199. 


Reading  through  a  Brick. 


The  apparatus  consists  of  five  short  tubes  jointed  at  right-an- 
gles, and  containing  four  mirrors  placed  at  the  angle  of  45°  to 


53.  Explain  by  Fig.  199,  how  a  candle  can  apparently  be  seen  through  a  brick. 
11 


243  REFRACTION    BY    LENSES. 

the  incident  rays  of  light,  flowing,  first,  from  the  object  to  be 
seen,  and  then  from  one  mirror  to  the  other.  Thus  the  rays 
from  tho  object  P,  pass  to  the  mirror  L,  through  the  tube  D  C, 
and  from  L  are  reflected  to  H,  anc\  from  H,  horizontally,  to  G, 
and  from  G,  vertically,  to  K,  and  lastly,  from  K  horizontally  to 
the  eye. 

Now  the  angle  of  reflection,  being  equal  to  that  of  incidence, 
each  mirror  has  the  effect  to  change  the  direction  of  the  rays  of 
light  equal  to  that  of  a  quarter  of  a  circle,  and  the  four  mirrors, 
therefore,  produce  a  change  equal  to  that  of  an  entire  circle. 


REFRACTION    BY    LENSES. 


54.  A  Lens  is  a  transparent  body,  generally  made  of  glass, 
and  so  shaped  that  the  rays  of  light  in  passing  through  it  are 
either  collected  together  or  dispersed.     Lens  is  a  Latin  word, 
which  comes  from  lentile,  a  small  flat  bean. 

It  has  already  been  shown,  that  when  the  rays  of  light  pass 
from  a  rarer  to  a  denser  medium,  they  are  refracted,  or  bent 
out  of  their  former  course,  except  when  they  happen  to  fall  per- 
pendicularly on  the  surface  of  the  medium.  (19.) 

-  The  point  where  no  refraction  is  produced  on  perpendicular 
rays,  is  called  the  axis  of  the  lens,  which  is  a  right  line  passing 
through  its  center,  and  perpendicular  to  both  its  surfaces,  as  op. 

In  every  beam  of  light  the  middle  ray  is  called  its  axis. 

Rays  of  light  are  said  to  fall  directly  upon  a  lens,  when  their 
axes  coincide  with  the  axis  of  the  lens ;  otherwise  they  are  said 
to  fall  obliquely. 

The  point  at  which  the  rays  of  the  sun  are  collected,  by  pass- 
ing through  a  lens,  is  called  the  principal  focus  of  that  lens. 

55.  Lenses  are  of  various  kinds,  and  have  received  certain 
names,  depending  on  their  shapes.     The  different  kinds  are 
shown  at  Fig.  200. 

A  prism,  seen  at  A,  has  two  plane  surfaces,  A  R,  and  A  S, 
inclined  to  each  other. 

A  plane  glass,  shown  at  B,  has  two  plane  surfaces,  parallel  to 
each  other. 

A  spherical  lens,  C,  is  a  ball  of  glass,  and  has  every  part  of 
its  surface  at  an  equal  distance  from  the  center. 

A  double-convex  lens,  D,  is  bounded  by  two  convex  surfaces, 
opposite  to  each  other. 

54.  What  is  a  lens?  What  is  the  axis  of  a  lens?  In  what  part  of  a  lens  IP  no  re- 
fraction produced  7  Where  is  the  axis  of  a  beam  of  light  ?  When  are  rays  of  light 
said  to  fall  directly  upon  a  lens?  How  many  kinds  of  lenses  are  mentioned?  55. 
What  are  the  names  and  shapes  of  each  1 


- 


CONVEX    LENfl. 


243 


Lenses  of  Various  Forma. 

A  plano-convex  lens,  E,  is  bounded  by  a  convex  surface  on 
one  side,  and  a  plane  one  on  the  other. 

A  double-concave  lens,  F,  is  bounded  by  two  concave  spher- 
ical surfaces,  opposite  to  each  other. 

A  plano-concave  lens,  G,  is  bounded  by  a  plane  surface  on 
one  side  and  a  concave  one  on  the  other. 

A  meniscus,  H,  is  bounded  by  one  concave,  and  one  convex 
spherical  surface,  which  two  surfaces  meet  at  the  edge  of  the 
lens. 

A  concavo-convex  lens,  I,  is  bounded  by  a  concave,  and  con- 
vex surface,  but  which  diverge  from  each  other,  if  continued. 

The  effects  of  the  prism  on  the  rays  of  light  will  be  shown  in 
another  place.  The  refraction  of  the  plane  glass  bends  the 
parallel  rays  of  light  equally  toward  the  perpendicular,  as  already 
shown.  The  sphere  is  not  often  employed  as  a  lens,  since  it  is 
inconvenient  in  use. 


CONVEX    LENS. 

56.   The  effect  of  the  convex  lens,  by  increasing  the  vimal 
angle,  is  to  magnify  all  objects  seen  through  it. 

Focal  Distance. — The 

focal  distances  of  convex  FIG> 

lenses,  depend  on  their  de- 
grees of  convexity.  The 
focal  distance  of  a  single, 
or  plano-convex  lens,  is 
the  diameter  of  a  sphere, 
of  which  it  is  a  section. 

If  the  whole  circle, 
Fig.  201,  be  considered 
the  circumference  of  a 
sphere,  of  which  the  pla- 
no-convex lens  B  A,  is  a 


Piano- Convex  Lent. 


244 


CONVEX    LENS. 


section,  then  the  focus  of  parallel  rays,  or  the  principal  focus, 
will  be  at  the  opposite  side  of  the  sphere,  or  at  C. 

57.  The  focal  distance  of  a  double-convex  lens,  is  the  radius, 
or  half  the  diameter  of  the  sphere,  of  which  it  is  a  part.     Hence 
the  plano-convex  lens,  being  one  half  of  the  double-convex  lens, 
the  latter  has  twice  the  refractive  power  of  the  former ;  for  the 
rays  suffer  the  same  degree  of  refraction  in  passing  out  of  the 
one  convex  surface,  that  they  do  in  passing  into  the  other. 

58.  Double-Convex  Lens. — The  shape  of  the  double-convex 
lens,  D  C,  Fig.  202,  is  that  of  two  plano-convex  lenses,  placed 
with  their  plane  surfaces 

in  contact,  and  conse- 
qiiently  the  focal  distance 
of  this  lens  is  nearly  the 
center  of  the  sphere  of 
which  one  of  its  surfaces 
is  a  part.  If  parallel 
rays  fall  on  a  convex  lens, 
it  is  evident  that  the  ray 
only,  which  penetrates 
the  axis  and  passes  to- 
ward the  center  of  the 

sphere,  will  proceed  with-  Double-Convex  Lens. 

out  refraction,  as  shown 

in  the  above  figures.  All  the  others  will  be  refracted  so  as  to 
meet  the  perpendicular  ray  at  a  greater  or  less  distance,  de- 
pending on  the  convexity  of  the  lens. 

59.  DIVERGING   RAYS  ON  A  CONVEX  LENS. — If  diverging 
rays  fall  on  the  surface  of  this  lens,  they  will,  by  refraction,  be 
rendered  less  divergent,  parallel,  or  convergent,  according  to  the 
degrees  of  their  divergence,  and  the  convexity  of  the  surface  of 
the  lens. 

Thus,  the  diverging  rays  1,  2,  <fec.,  Fig.  203,  are  refracted 
by  the  lens  A  0,  in  a  degree  just  sufficient  to  render  them  par- 
allel, and  therefore,  would  pass  off  in  right  lines,  indefinitely,  or 
without  ever  forming  a  focus. 

It  is  obvious  by  the  same  law,  that  were  the  rays  less  diverg- 
ent, or  were  the  surface  of  the  lens  more  convex,  the  rays  in 


56.  What  is  the  effect  of  the  convex  lens?  On  what  do  the  focal  distances  of  con- 
vex lenses  depend  1  57.  What  is  the  focal  distance  of  any  plano-convex  lens  ?  What 
is  the  focal  distance  of  the  double-convex  lens?  58.  What  is  l  he  shape  of  the  double- 
convex  lens  7  59.  How  are  divergent  rays  affected  bypassing  through  a  convex 
lens  7  What  is  its  effect  on  parallel  rays?  What  is  its  effect  on  converging  rays1 
What  kind  of  leases  are  spectacle  glasses  for  old  people  ? 


CONVEX   LEWS.  245 

Fig.  203,  would  be*  nG  **• 

come  convergent, 
instead  of  parallel, 
because  the  same 
retractive  power 
which  would  render 
divergent  rays  par- 
allel, would  make 
parallel  rays  con- 
vergent, and  con-  o 

verging     rays      Still  Diverging  Rays. 

more  convergent. 

Thus  the  pencils  of  converging  rays,  Fig.  204,  are  rendered 
still  more  convergent  by  their  passage  through  the  lens,  and 
are  therefore  brought  to  a  focus  nearer  the  lens,  in  proportion 
to  their  previous  convergence. 

The  eyeglasses  of  spectacles  for  old  people  are  double-convex 
lenses,  more  or  less  spherical,  according  to  the  age  of  the  person, 
or  the  magnifying  power  required. 

60.  BURNING  GLASS. — The  common  burning  glasses,  which 
are  used  for  lighting  cigars,  and  sometimes  for  kindling  fires, 
are  also  convex  lenses.     Their  effect  is  to  concentrate  to  a  focus, 
or  point,  the  heat  of  the  sun  which  falls  on  their  whole  surface; 
and  hence  the  intensity  of  their  effects  is  in  proportion  to  the 
extent  of  their  surfaces,  and  their  focal  lengths. 

61.  VISUAL  ANGLE. — It  has  been  explained,  that  the  reason 
why  the  convex  mirror  diminishes  the  images  of  objects  is,  that 
the  rays  which  come  to  the  eye  from  the  extreme  parts  of  the 
object  are  rendered  less  convergent  by  reflection,  from  the  con- 
vex surface,  and  that  in  consequence,  the  angle  of  vision  is  madft 
more  acute.  (41.) 

62.  Now,  the  refractive  power  of  the  convex  lens  has  exactly 
the  contrary  effect,  since  by  converging  the  rays  flowing  from 
the  extremities  of  an  object,  the  visual  angle  is  rendered  more 
obtuse,  and  therefore  all  objects  seen  through  it  appear  magnified. 

63.  Suppose  the  object  A,  Fig.  205,  appears  to  the  naked 
eye  of  the  length  represented  in  the  drawing.     Now,  as  the 
rays  coming  from  each  end  of  the  object,  form  by  their  con- 
vergence at  the  eye,  the  visual  angle,  or  the  angle  under  which 
the  object  is  seen,  and  we  call  objects  large  or  small  in  propor- 

60.  What  kind  of  a  lens  is  a  burning  glass!  61.  What  is  the  visual  angle?  62. 
What  is  the  effect  of  the  convex  lens  on  the  visual  angle  ?  63.  Why  does  the  same 
object,  when  at  a  distance,  appear  smaller  than  when  near  ?  Why  does  an  object 
appear  larger  through  the  convex  lens  than  otherwise  1 


246 


CONVEX    LENS. 


FIG.  204. 


FIG.  208. 


Converging  Ray. 


FIG.  205. 


Visual  Angle. 


Visual  Angle. 

tion  as  this  angle  is  obtuse  or  acute,  if,  therefore,  the  object  A. 
be  withdrawn  further  from  the  .eye,  it  is  apparent  that  the  rays 
O  O,  proceeding  from  its  extremities,  will  enter  the  eye  under  a 
more  acute  angle,  and  therefore  that  the  object  will  appear  di- 
minished in  proportion.  This  is  the  reason  why  things  at  a 
distance  appear  smaller  than  when  near  us.  When  near,  the 
visual  angle  is  increased,  and  when  at  a  distance  it  is  diminished. 

The  effect  of  the  convex  lens  is  to  increase  the  visual  angle, 
by  bending  the  rays  of  light  coming  from  the  object,  so  as  to 
make  them  meet  at  the  eye  more  obtusely ;  and  hence  it  has 
the  same  effect,  in  respect  to  the  visual  angle,  as  bringing 
the  object  nearer  the  eye.  This  is  shown  by  Fig.  206,  where 
it  is  obvious,  that  did  the  rays  flowing  from  the  extremities  of 
the  arrow  meet  the  eye  without  refraction,  the  visual  angle 
would  be  less,  and  therefore  the  object  would  appear  shorter. 
Another  effect  of  the  convex  lens,  is  to  enable  us  to  see  objects 
nearer  the  eye  than  without  it,  as  will  be  explained  under  the 
article  Vision. 

Now,  as  the  rays  of  light  flow  from  all  parts  of  a  visible  ob- 
ject of  whatever  shape,  so  the  breadth,  as  well  as  the  length,  is 
increased  by  the  convex  lens,  and  thus  the  whole  object  appears 
magnified. 

64.  CONCAVE  LENS. — The  effect  of  the  concave  lens  is  di- 


64.  What  is  the  effect  of  the  concave  lens  7 


CONVEX    LENS. 


247 


rectly  opposite  to  that  of  the  convex.  In  other  terras,  by  a  con- 
cave lens,  parallel  rays  are  rendered  diverging,  converging  rays 
have  their  convergence  diminished,  and  diverging  rays  have  their 
divergence  increased,  according  to  the  concavity  of  the  lens. 

These  glasses,  therefore,  exhibit  things  smaller  than  they 
really  are,  for  by  diminishing  the  convergence  of  the  rays  com- 
ing from  the  extreme  points  of  an  object,  the  visual  angle  is 
rendered  more  acute,  and  hence  the  object  appears  diminished 
by  this  lens,  for  the  opposite  reason,  that  it  is  increased  by  the 
convex  lens.  This  will  be  made  plain  by  the  two  following 
diagrams. 

Suppose  the  object  A  B,  Fig.  207,  to  be  placed  at  such  a 
distance  from  the  eye,  as  to  give  the  rays  flowing  from  it,  the 
degrees  of  convergence  represented  in  the  figure,  and  suppose 
that  the  rays  enter  the  eye  under  such  an  angle  as  to  make  the 
object  appear  two  feet  in  length. 


FIG.  207. 


FIG.  203. 


Natural  Vision. 


Piano- Concave  Lent. 


Now,  the  length  of  the  same  object,  seen  through  the  con- 
cave lens,  Fig.  208,  will  appear  diminished,  because  the  rays 
coming  from  it  are  bent  outward,  or  made  less  convergent  by 
refraction,  as  seen  in  the  figure,  and  consequently  the  visual 
angle  is  more  acute  than  when  the  same  object  is  seen  by  the 
naked  eye.  Its  length,  therefore,  will  appear  less  in  proportion 
as  the  rays  are  rendered  less  convergent. 

The  spectacle  glasses  of  short-sighted  people  are  concave 
lenses,  by  which  the  images  of  objects  are  formed  further  back 
in  the  eye  than  otherwise,  as  will  be  explained  under  the  next 
article. 


What  effect  does  this  lens  have  upon  parallel,  diverging,  and  converging  rays? 
Why  do  objects  appear  smaller  through  this  glass  than  they  do  to  the  naked  eye? 
Explain  Figs.  207  and  203.  and  show  the  reason  why  the  same  object  appears  smaller 
through  208.  What  defect  in  the  eye  requires  concave  leases? 


248 


VISION. 


Human  Eye. 


VISION. 

65.  In  the  application  of  the  principles  of  optics  to  the  ex- 
plonation  of  natural  phenomena,  it  is  necessary  to  give  a  descrip- 
tion of  the  most  perfect  of  all  optical  instruments,  the  eye. 

Human    Eye. — Fig. 

209  is  a  vertical  section  FIG-  209- 

of  the  human  eye.  Its 
form  is  nearly  globular, 
with  a  slight  projection 
or  elongation  in  front. 
It  consists  of  four  coats, 
or  membranes ;  namely, 
the  sclerotic,  the  cornea, 
the  choroid,  and  the 
retina.  It  has  two  fluids 
confined  within  these 
membranes,  called  the 
aqueous,  and  the  vitre- 
ous humors,  and  one  lens,  called  the  crystaline.  The  sclerotic 
coat  is  the  outer  and  strongest  membrane,  and  its  anterior  part 
is  well  known  as  the  white  of  the  eye.  This  coat  is  marked  in 
the  figure  a  a  a  a.  It  is  joined  to  the  corpea  b  b,  which  is 
the  transparent  membrane  in  front  of  the  eye,  through  which 
we  see.  The  choroid  coat  is  a  thin,  delicate  membrane,  which 
lines  the  sclerotic  coat  on  the  inside.  On  the  inside  of  this  lies 
the  retina,  d  d  d  d,  which  is  the  innermost  coat  of  all,  and  is 
an  expansion,  or  continuation  of  the  optic  nerve  o.  This  ex- 
pansion of  the  optic  nerve  is  the  immediate  seat  of  vision.  The 
iris,  o  o,  is  seen  through  the  cornea,  and  is  a  thin  membrane, 
or  curtain,  of  different  colors  in  different  persons,  and  therefore 
gives  color  to  the  eyes.  » In  black-eyed  persons  it  is  black,  in 
blue-eyed  persons  it  is  blue,  &c.  Through  the  iris,  is  a  circular 
opening,  called  the  pupil,  which  expands  or  enlarges  when  the 
light  is  faint,  and  contracts  when  it  is  too  strong.  The  space 
between  the  iris  and  the  cornea  is  called  the  anterior  chamber 
of  the  eye,  and  is  filled  with  the  aqueous  humor,  so  called  from 
its  resemblance  to  water.  Behind  the  pupil  and  iris  is  situated 

65.  What  is  the  most  perfect  of  all  optical  instruments  ?  What  is  the  form  of  the 
numan  eye  ?.  How  many  coats  or  membranes  has  the  eye  1  What  are  they  called? 
How  maiiy  fluids  has  the  eye,  and  what  are  they  called  7  What  is  the  lens  of  the  eye 
called  ?  What  coat  forms  the  white  of  the  eye?  Describe  wher.e  the  several  coats 
and  humors  are  situated.  What  is  the  iris]  What  is  the  retinal  Where  is  the 
sense  of  vision  ?  What  is  the  design  of  Fig.  210  J  What  is  said  concerning  the  small 
number  of  the  rays  which  enter  the  eye  from  a  visible  object  1 


VISION. 


249 


FIG.  210. 


the  crystaline  lens  e,  which  is  a  firm  and  perfectly  transparent 
body,  through  which  the  rays  of  light  pass  from  the  pupil  to 
the  retina.  Behind  the  lens  is  situated  the  posterior  chamber 
of  the  eye,  which  is  filled  with  the  vitreous  humor,  v  v.  This 
humor  occupies  much  the  largest  portion  of  the  whole  eye,  and 
on  it  depends  the  shape  and  permanence  of  the  organ. 

From  the  above  description  of  the  eye  it  will  be  easy  to  trace 
the  progress  of  the  rays  of  light  through  its  several  parts,  and 
to  explain  in  what  manner  vision  is  performed. 

In  doing  this,  we  must  keep  in  mind  that  the  rays  of  light 
proceed  from  every  part  and  point  of  a  visible  object,  as  here- 
tofore stated,  and  that  it  is  necessary  only  for  a  few  of  the  rays, 
when  compared  with  the  whole  number,  to  enter  the  eye,  in 
order  to  make  the  object  visible. 

Thus,  the  object  A  B,  Fig. 
210,  being  placed  in  the 
li^ht,  sends  pencils  of  rays  in 
all  possible  directions,  some 
of  which  will  strike  the  eye 
in  any  position  where  it  is  vis- 
ible. These  pencils  of  rays 
not  only  flow  from  the  points 
designated  in  the  figure,  but 
in  the  same  manner  from 
every  other  point  on  the  sur- 
face of  a  visible  object.  To 
render  an  object  visible,  there- 
fore, it  is  only  necessary  that 
the  eye  should  collect  and 
concentrate  a  sufficient  num- 
ber of  these  rays  on  the  retina, 
to  form  its  image  there,  and 
from  this  image  the  sensation 
of  vision  is  excited. 

66.  From  the  luminous  body  L,  Fig.  211,  the  pencils  of  rays 
flow  in  all  directions,  but  it  is  only  by  those  which  enter  the 
pupil,  that  we  gain  any  knowledge  of  its  existence;  and  even 
these  would  convey  to  the  mind  no  distinct  idea  of  the  object, 
unless  they  were  refracted  by  the  humors  of  the  eye,  for  did 
these  rays  proceed  in  their  natural  state  of  divergence  to  the 


Pencils  of  Rays. 


66.  Explain  the  design  of  Fig.  211.    Why  would  not  the  ravs  of  light  give  a  distinct 
Idea  of  the  object,  without  refraction  by  the  humors  of  tbe  eye  1 

11* 


250 


Refraction  by  the  Eye. 

retina,  the  image  there  formed  would  be  too  extensive,  and  con- 
sequently too  feeble  to  give  a  distinct  sensation  of  the  object. 

It  is,  therefore,  by  the  refracting  power  of  the  aqueous  hu- 
mor, and  of  the  crystaline  lens,  that  the  pencils  of  rays  are  so 
concentrated  as  to  form  a  perfect  picture  of  the  object  on  the 
retina. 

67.  Inverted  Image  on  the  Retina. — We  have  already  seen, 
that  when  the  rays  of  light  are  made  to  cross  each  other  by  re- 
flection from  the  concave  mirror,  the  image  of  the  object  is  in- 
verted ;  the  same  happens  when  the  rays  are  made  to  cross 
each  other  by  refraction  through  a  convex  lens.     This,  indeed, 
must  be  a  necessary  consequence  of  the  intersection  of  the  rays  ; 
for  as  light  proceeds  in  straight  lines,  those  rays  which  come 
from  the  lower  part  of  an  object,  on  crossing  those  which  come 
from  its  upper  part,  will  represent  this  part  of  the  picture  on 
the  upper  half  of  the  retina,  and,  for  the  same  reason,  the  up- 
per part  of  the  object  will  be  painted  on  the  lower  part  of  the 
retina. 

Now,  all  objects  are  represented  on  the  retina  in  an  inverted 
position  ;  that  is,  what  we  call  the  upper  end  of  a  vertical  ob- 
ject, is  the  lower  end  of  its  picture  on  the  retina,  and  so  the 
contrary. 

68.  Eye  of  an  Ox. — This  is  readily  proved  by  taking  the  eye 
of  an  ox,  and  cutting  away  the  sclerotic  coat,  so  as  to  make  it 
transparent  on  the  back  part,  next  the  vitreous  humor.     If  now 
a  piece  of  white  paper  be  placed  on  this  part  of  the  eye,  the 
images  of  objects  will  appear  figured  on  the  paper  in  an  in- 
verted position.     The  same  effect  will  be  produced  on  looking 
at  things  through   an  eye  thus  prepared ;  they  will   appear 
inverted. 


67.  Explain  how  it  is  that  the  images  of  objects  are  inverted  on  the  retina.  6S 
What  experiment  proves  that  th«  images  of  objects  are  inverted  on  the  retina  7  Ex 
riiain  Fitf.  212. 


251 


Inversion  of  Objects. 

The  actual  position  of  the  vertical  object  A,  Fty.  212,  as 
painted  on  the  retina,  is  therefore  such  as  is  represented  by  the 
figure.  The  rays  from  its  upper  extremity,  coming  in  divergent 
lines,  are  converged  by  the  crystaline  lens,  and  fall  on  the  retina 
at  O ;  while  'those  from  its  lower  extremity,  by  the  same  law, 
fall  on  the  retina  at  C,  the  rays  crossing  each  other  as  they  pass 
the  humors  of  the  eye.  . 

69.  In  order  that  vision  may  be  perfect,  it  is  necessary  that 
the  images  of  objects  should  be  formed  precisely  on  the  retina, 
and  consequently,  if  the  refractive  power  of  the  eye  be  too  small, 
or  too  great,  the  image  will  not  fall  exactly  on  the  seat  of  vision, 
but  will  be  formed  either  before,  or  tend  to  form  behind  it.     In 
both  cases,  perhaps,  an  outline  of  the  object  may  be  visible,  but 
it  will  be  confusrd  and  indistinct. 

70.  Cornea  too  Prominent. — If  the  cornea  is  too  convex,  or 
prominent,  the  image  will  be  formed  before  it  reaches  the  retina, 
for  the  same  reason,  that  of  two  lenses,  that  which  is  most  con- 
vex will  have  the  least  focal  distance.     Such  is  the  def  ct  in  the 
eyes  of  persons  who  are  short-sighted,  and  honce  the  necessity 
of  their  bringing  objects  as  near  the  eye  as  possible,  so  as  to 
make  the  rays  converge  at  the  greatest  distance  behind  the 
crystaline  lens. 

The  effect  of  uncommon  convexity  in  the  cornea  on  the  rays 
of  light,  is  shown  at  Fig.  213,  where  it  will  be  observed  that 
the  image,  instead  of  being  formed  on  the  retina  R,  is  suspended 
in  the  vitreous  humor,  in  consequence  of  there  being  too  gn  at 
a  refractive  power  in  the  eye.  It  is  hardly  necessary  to  say, 
that  in  this  case,  vision  must  be  very  imperfectly  performed. 

This  defect  of  sight  is  remedied  by  spectacles,  the  glasses  of 

69.  Suppose  the  refractive  power  of  the  eye  is  too  great,  or  too  little,  why  will  TI'SJOD 
be  imperfect  ?  70  If  the  cornea  is  too  convex,  where  wift  the  image  be  formed* 
How  is  the  sight  improved,  when  the  cornea  is  too  convex  1  How  do  such  lenses  act 
to  improve  the  Bight  1 


252 


Cornea  too  Convex. 

which  are  concave  lenses.  Such  glasses,  by  rendering  the  rays 
of  light  less  convergent,  before  they  reach  the  eye,  counteract 
the  too  great  convergent  power  of  the  cornea  and  lens,  and  thus 
throw  the  image  on  the  retina. 

71.  Cornea  too  Flat. — If,  on  the  contrary,  the  humors  of  the 
eye,  in  consequence  of  age,  or  any  other  cause,  have  become 
less  in  quantity  than  ordinary,  the  eyeball  will  not  be  suffi- 
ciently distended,  and  the  cornea  will  become  too  flat,  or  not  suf- 
ficiently convex,  to  make  the  rays  of  light  meet  at  the  proper 
place,  and  the  image  will  therefore  tend  to  be  formed  beyond 
the  retina,  instead  of  before  it,  as  in  the  other  case.  Hence, 
aged  people,  who  labor  under  this  defect  of  vision,  can  not  see 
distinctly  at  ordinary  distances,  but  are  obliged  to  remove  the 
object  as  far  from  the  eye  as  possible,  so  as  to  make  its  refrac- 
tive power  bring  the  image  within  the  seat  of  vision. 


G.  214. 


Cornea  too  Flat. 

The  defect  arising  from  this  cause  is  represented  by  Fig.  214, 

71.  Where  do  the  rays  tend  to  meet  when  the  cornea  is  not  sufficiently  convex? 
How  is  vision  assisted  when  the  eye  wants  convexity?  How  do  convex  lenses  help 
the  sight  of  aged  persons  ? 


VISION.  253 

where  it  will  be  observed  that  the  image  is  formed  behind  the 
retina,  showing  that  the  convexity  of  the  cornea  is  not  sufficient 
to  bring  the  image  within  the  seat  of  distinct  vision.  This  im- 
perfection of  sight  is  common  to  aged  persons,  and  is  corrected 
in  a  greater  or  less  degree  by  double-convex  lenses,  such  as  the 
common  spectacle  glasses.  Such  glasses,  by  causing  the  rays 
of  light  to  converge,  before  they  meet  the  eye,  assist  the  refrac- 
tive power  of  the  crystaline  lens,  and  thus  bring  the  focus,  or 
image,  within  the  sphere  of  vision. 

72.  Why  we  see  Objects  Erect. — It  has  been  considered  dif- 
ficult to  account  for  the  reason  why  we  see  objects  erect,  when 
they  are  painted  on  the  retina  inverted,  and  many  learned  theo- 
ries  have   been   written   to  explain   this  fact.     But  it  is  most 
probable  that  this  is  owing  to  habit,  and  that  the  image,  at  the 
bottom  of  the  eye,  has  no  relation  to  the  terms  above  and  be- 
low, but  to  the  position  of  our  bodies,  and  other  things  which 
surround  us.     The  term  perpendicular,  and  the  idea  which  it 
conveys  to  the  mind,  is  merely  relative ;  but  when  applied  to 
an  object  supported  by  the  earth,  and  extending  toward  the 
skies,  we  call  the  body  erect,  because  it  coincides  with  the  posi- 
tion of  our  own  bodies,  and  we  see  it  erect  for  the  same  reason. 
Had  we  been  taught  to  read  by  turning  our  books  upside  down, 
what  we  now  call  the  tfpper  part  of  the  book  would  have  been 
its  under  part,  and  that  reading  would  have  been  as  easy  in 
that  position  as  in  any  other,  is  plain  from  the  fact  that  printers 
read  their  type,  when  set  up,  as  readily  as  they  do  its  impres- 
sions on  paper. 

ANGLE    OF   VISION. 

73.  This  subject,  partly  explained,  needs  further  illustration. 
The  angle  under  which  the  rays  of  light,  coming  from  the 

extremities  of  an  object,  cross  each  other  at  the  eye,  bears  a  pro- 
portion directly  to  the  length,. and  inversely  to  the  distance  of 
the  object. 

Suppose  the  object  A  B,  Fig.  215,  to  be  four  feet  long,  and 
to  be  placed  ten  feet  from  the  eye,  then  the  rays  flowing  from 
its  extremities,  would  intersect  each  other  at  the  eye,  under  a 
given  angle,  which  will  always  be  the  same  when  the  object  is 
at  the  same  distance.  If  the  object  be  gradually  moved  toward 

72.  Why  do  we  see  things  erect,  when  the  images  are  inverted  on  the  retina?  73. 
What  is  the  visual  angle  1  How  may  the  visual  angle  of  the  same  object  be  increased 
or  diminished  1  When  do  objects  of  different  magnitudes  form  the  same  visual  an» 
gle  1  Explain  Fig.  215. 


254 


VISION. 
FIG.  215. 


Angle  of  Vision. 

the  eye,  to  the  place  C  D,  then  the  angle  will  be  gradually  in- 
creased in  quantity,  and  the  object  will  appear  larger,  since  its 
image  on  the  retina  will  be  increased  in  length  in  the  propor- 
tion as  the  lines  I  I,  are  wider  apart  than  O  O.  On  the  con- 
trary, were  A  B  removed  to  a  greater  distance  from  the  first 
position,  it  is  obvious  that  the  angle  would  be  diminished  in 
proportion. 

The  lines  thus  proceeding  from  the  extremities  of  an  object, 
and  representing  the  rays  of  light,  form  an  angle  at  the  eye, 
which  is  called  the  visual  angle,  or  the  angle  under  which  things 
are  seen.  The  lines  A  N  B,  therefore,  form,  one  visual  angfe, 
and  the  lines  C  N  D  another  visual  angle. 

We  see  from  this  investigation,  that  the  apparent  magnitude 
of  objects  depending  on  the  angles  of  vision,  will  vary  according 
to  their  distances  from  the  eye,  and  that  these  magnitudes  di- 
minish in  a  proportion  inversely  as  their  distances  increase. 

74.  How  WE  JUDGE  OF  MAGNITUDES. — In  the  apparent  mag- 
nitude of  objects  seen  through  a  lens,  or  when  their  images 
reach  the  eye  by  reflection  from  a  mirror,  our  senses  are  chiefly, 
if  not  entirely,  guided  by  the  angle  of  vision.  In  forming  our 
judgment  of  the  sizes  of  distant  objects,  whose  magnitudes  were 
before  unknown,  we  are  also  guided  more  or  less  by  the  visual 
angle,  though  in  this  case  we  do  not  depend  entirely  on  the 
sense  of  vision.  Thus,  if  we  see  two  balloons  floating  in  the 
air,  one  of  which  is  larger  than  the  other,  we  judge  of  their 
comparative  magnitudes  by  the  difference  in  their  visual  angles, 
and  of  their  real  magnitudes  by  the  same  angles,  and  the  dis- 
tance we  suppose  them  to  be  from  us. 

74.  How  do  we  judge  of  the  magnitudes  of  distant  objects  ?    Under  what  circum< 
etantes  IB  our  riense  of  vieltfn  guided  entirely  By  the  visual  angle  t 


VISION.  255 

75.  But  when  the  object  is  near  us,  and  seen  with  the  naked 
eye,  we  then  judge  of  the  magnitude  by  our  experience,  and  not 
entirely  by  the  visual  angle.     Thus,  the  't^ree  arrows,  A  E  M, 
Fig.  215,  all  of  them  make  the  same  angle  on  the  eye,  and  yet 
we  know,  by  further  examination,  that  they  are  all  of  different 
lengths.     And  so  the  two  arrows,  A  B,  and  C  D,  though  seen 
under  different  visual  angles,  will  appear  of  the  same  size,  be- 
cause experience  has  taught  us  that  this  difference  depends  only 
on  the  comparative  distance  of  the  two  objects. 

76.  As  the  visual  angle  diminishes  inversely  in  proportion  as 
the  distance  of  the  object  increases,  so  when  the  distance  is  so 
great  as  to  make  the  angle  too  minute  to  be  perceptible  to  the 
eye,  then  the  object  becomes  invisible.     Thus,  when  we  watch 
an  eagle  flying  from  us,  the  angle  of  vision  is  gradually  dimin- 
ished, Until  the  rays  proceeding  from  the  bird  form  an  image  on 
the  retina  too  small  to  excite  sensation,  and  then  we  say  the 
eagle  has  flown  out  of  sight. 

The  same  principle  holds  with  respect  to  objects  which  are 
near  the  eye,  but  are  too  small  to  form  an  image  on  the  retina 
which  is  perceptible  to  the  senses.  Such  objects  to  the  naked 
eye,  are  of  course  invisible,  but  when  the-  visual  angle  is  en- 
larged, by  means  of  the  convex  lens,  they  become  visible ;  that 
is,  their  images  on  the  retina  excite  sensation. 

77.  SIZE  OF  THE  IMAGE  ON  THE  RETINA. — The  actual  size 
of  an  image  on  the  retina,  capable  of  exciting  sensation,  and 
consequently  of  producing  vision,  may  be  too  small  for  us  to 
appreciate  by  any  of  our  other  senses ;  for  when  we  consider 
how  much  smaller  the  image  must  be  than  the  object,  and  that 
a  human  hair  can  be  distinguished  by  the  naked  eye  at  the  dis- 
tance of  twenty  or  thirty  feet,  we  must  suppose  that  the  retina 
is  endowed  with  the  most  delicate  sensibility,  to  be  excited  by 
a  cause  so  minute.     It  has  been  estimated  that  the  image  of  a 
man,  on  the  retina,  seen  at  the  distance  of  mile,  is  not  more 
than  the  five  thousandth  part  of  an  inch  in  length. 

78.  INDISTINCT  VISION. — On  the  contrary,  if  the  object  be 
brought  too  near  the  eye,  its  image  becomes  confused  and  in- 
distinct, because  the  rays  flowing  from  it,  fall  on  the  crystaline- 
lens  in  a  state  too  divergent  to  be  refracted  to  a  focus'on  the 
retina. 


75.  How  do  we  judge  of  the  comparative  size  of  objects  near  us  ?  76.  When  does 
a  retreat  n?  object  become  invis  ble  to  the  eye  1  How  does  a  convex  lens  act  to  make 
us  see  objects  which  are  invisible  without  it)  77.  What  is  said  uf  the  actual  size  of 
an  image  on  the  retina  ?  78.  Why  are  objects  indistinct  when  brought  too  near  the 
eye? 


256 


OPTICAL    INSTRUMENTS. 


Indistinct  Vision. 


This  will  be  apparent  by 
Fig.  216,  where  we  sup- 
pose that  the  object  "A,  is 
brought  within  an  inch  or 
two  of  the  eye,  and  that 
the  rays  proceeding  from 
it  enter  the  pupil  so  ob- 
liquely as  not  to  be  refract- 
ed by  the  lens,  so  as  to  form 
a  distinct  image. 

Could  we  see  objects  dis- 
tinctly at  the  shortest  distance,  we  should  be  able  to  examine 
things  that  are  now  invisible,  since  the  visual  angle  would  then 
be  increased,  and  consequently  the  image  on  the  retina  enlarged, 
in  proportion  as  objects  were  brought  near  the  eye. 

79.  This  is  proved  by  intercepting  the  most  divergent  rays; 
in  which  case  an  object  may  be  brought  near  the  eye,  and  will 
then  appear  greatly  magnified.  Make  a  small  orifice,  as  a  pin- 
hole,  through  a  piece  of  dark-colored  paper,  and  then  look 
through  the  orifice  at  small  objects,  such  as  the  letters  of  a 
printed  book.  The  letters  will  appear  much  magnified.  The 
rays,  in  this  case,  are  refracted  to  a  focus,  on  the  retina,  because 
the  small  orifice  prevents  those  which  are  most  divergent  from 
entering  the  eye,  so  that  notwithstanding  the  nearness  of  the 
object,  the  rays  which  form  the  image  are  nearly-parallel. 


OPTICAL   INSTRUMENTS. 

80.  SINGLE  MICROSCOPE. — The  principle  of  the  single  micro- 
scope, or  convex  lens,  will  be  readily  understood,  if  the  pupil 
will  remember  what  has  been  said  on  the  refraction  of  lenses,  in 
connection  with  the  facts  just  stated.  For,  the  reason  why  ob- 
jects appear  magnified  through  a  convex  lens,  is  not  only  be- 
cause the  visual  angle  is  increased,  but  because  when  brought 
near  the  eye,  the  diverging  rays  from  the  object  are  rendered 
parallel  by  the  lens,  and  are  thus  thrown  into  a  condition  to  be 
brought  to  a  focus  in  the  proper  place  by  the  humors  of  the 
eye. 

Let  A,  Fig.  217,  be  the  distance  at  which  an  object  can  bo 


Suppose  objects  could  be  seen  distinctly  within  an  inch  or  two  of  the  eye,  how 
would  their  dimensions  be  affected  1  79.  How  is  it  proved  that  objects  placed  near 
Jie  eye  are  magnified  7  How  does  a  small  orifice  enable  us  to  see  an  object  distinctly 
near  "the  eye?  80.  Why  does  a  convex  lens  make  an  object  distinct  when  near  the 
eve  (  Explain  Fig.  217.  How  are  the  most  powerful  single  microscopes  made? 


OPTICAL    INSTRUMENTS.  257 

FIG.  217. 


Single  Microscope. 


seen  distinctly,  and  B,  the  distance  at  which  the  same  object  is 
seen  through  the  lens,  and  suppose  the  distance  of  A  from  the 
eye,  be  twice  that  of  B.  Then,  because  the  object  is  at  half  the 
distance  that  it  was  before,  it  will  appear  twice  as  large ;  and 
had  it  been  seen  one- third,  one-fourth,  or  one-tenth  its  former 
distance,  it  would  have  been  magnified  three,  four,  or  ten  times, 
and  consequently  its  surface  would  be  increased  9,  16,  or  100 
times. 

The  most  powerful  single  microscopes  are  made  of  minute 
globules  of  glass,  which  are  formed  by  melting  the  ends  of  a 
few  threads  of  spun  glass  in  a  flame  of  alcohol.  Small  globules 
of  water  placed  in  an  orifice  through  a  piece  of  tin,  or  other 
thin  substance,  will  also  make  very  powerful  microscopes.  In 
these  minute  lenses,  the  focal  distance  is  only  a  tenth  or  twelfth 
part  of  an  inch  from  the  lens,  and  therefore  the  eye,  as  well 
as  the  object  to  be  magnified,  must  be  brought  very  near  the 
instrument. 

81.  COMPOUND  MICROSCOPE. — This  consists  of  two  convex 
lenses,  by  one  of  which  the  image  is  formed  within  the  tube  of 
the  instrument,  and  by  the  other  this  image  is  magnified  as 
seen  by  the  eye ;  so  that  by  this  instrument  the  object  itself  is 
not  seen,  as  with  the  single  microscope,  but  we  see  only  its 
magnified  image. 

The  small  lens  placed  near  the  object,  and  by  which  its  image 
is  formed  within  the  tube,  is  called  the  object  glass,  while  the 
larger  one,  through  which  the  image  is  seen,  is  called  the 
eyeglass. 

This  arrangement  is  represented  at  Fig.  218.  The  object  A 
is  placed  a  little  beyond  the  focus  of  the  object  glass  B,  by  which 
an  inverted  and  enlarged  image  of  it  is  formed  within  the  in- 

81.  How  many  lenses  form  the  compound  microscope  1  Which  is  the  object,  and 
which  the  eyeglass?  Is  the  object  seen  with  this  instrument,  or  only  its  image? 
Explain  Fig.  218,  and  show  where  the  image  is  formed  in  this  tube. 


258 


OPTICAL   INSTRUMENTS. 


Compound  Microscope. 


strument  at  C.  This  image  is  seen  through  the  eyeglass  D,  by 
which  it  is  again  magnified,  and  it  is  at  last  figured  on  the  retina 
in  its  original  position. 

These  glasses  are  set  in  a  case  of  brass,  the  object  glass  being 
made  to  take  out,  so  that  others  of  different  magnifying  powers 
may  be  used,  as  occasion  requires. 

82.  SOLAR  MICROSCOPE. — This  consists  of  two  lenses,  one  of 
which  is  called  the  condenser,  because  it  is  employed  to  con- 
centrate the  rays  of  the  sun,  in  order  to  illuminate  more  strongly 
the  object  to  be  magnified.  The  other  is  a  double-convex  lens, 
of  considerable  magnifying  power,  by  which  the  image  is  formed. 
In  addition  to  these  lenses,  there  is  a  plain  mirror,  or  piece  of 
common  looking-glass,  which  can  be  moved  in  any  direction, 
and  which  reflects  the  rays  of  the  sun  on  the  condenser. 

FIG.  219. 


Solar  Microscope. 

The  object  A,  Fig.  219,  being  placed  nearly  in  the  focus  of 

82.  How  many  lenses  has  the  solar  microscope?  Why  is  one  of  the  lenses  of  the 
solar  microscope  called  the  condenser  ?  Describe  the  uses  of  the  two  lenses  and  the 
reflector.  Is  the  object,  or  only  the  shadow,  seen  by  this  instrument  1 


TELESCOPE. 

the  condenser  B,  is  strongly  illuminated,  in  consequence  of  the 
rays  of  the  sun  being  thrown  on  B,  by  the  mirror  C.  The  ob- 
ject is  not  placed  exactly  in  the  focus  of  the  condenser,  because, 
in  most  cases,  it  would  be  soon  destroyed  by  its  heat,  and  be- 
cause the  focal  point  would  illuminate  only  a  small  extent  of 
surface,  but  may  be  exactly  in  the  focus  of  the  small  lens  D,  by 
which  no  such  accident  can  happen.  The  lines  O  O,  represent 
the  incident  rays  of  the  sun,  which  are  reflected  on  the  condenser. 

When  the  solar  microscope  is  used,  the  room  is  darkened, 
the  only  light  admitted  being  that  which  is  thrown  on  the  ob- 
ject by  the  condenser,  which  light  passing  through  the  small 
lens,  gives  the  magnified  shadow  E,  of  the  small  object  A,  on 
the  wall  of  the  room,  or  on  a  screen.  The  tube  containing  the 
two  lenses  is  passed  through  the  window  of  the  room,  the  re- 
flector remaining  outside. 

In  the  ordinary  use  of  this  instrument,  the  object  itself  is  not 
seen,  but  only  its  shadow  on  the  screen,  and  it  is  not  designed 
for  the  examination  of  opaque  objects. 

When  the  small  lens  of  the  solar  microscope  is  of  great  mag- 
nifying power,  it  presents  some  of  the  most  striking  and  curious 
of  optical  phenomena.  The  shadows  of  mites  from  cheese,  or 
figs,  appear  nearly  two  feet  in  length,  presenting  an  appearance 
exceedingly  formidable  and  disgusting ;  and  the  insects  from 
common  vinegar  appear  eight  or  ten  feet  long,  and  in  perpetual 
motion,  resembling  so  many  huge  serpents. 

TELESCOPE. 

83.  The  Telescope  is  an  optical  instrument,  employed  to  view 
distant  bodies,  and,  in  effect,  to  bring  them  nearer  the  eye,  by  in- 
creasing the  apparent  angles  under  which  such  objects  are  seen. 

These  instruments  are  of  two  kinds,  namely,  refracting  and 
reflecting  telescopes.  In  the  first  kind,  the  image  of  the  object 
is  seen  with  the  eye  directed  toward  it ;  in  the  second  kind,  the 
image  is  seen  by  reflection  from  a  mirror,  while  the  back  is  to- 
ward the  object,  or  by  a  double  reflection,  with  the  face  toward 
the  object. 

The  telescope  is  the  most  important  of  all  optical  instruments, 
since  it  unfolds  the  wonders  of  other  worlds,  and  gives  us  the 
means  of  calculating  the  distances  of  the  heavenly  bodies,  and 


83.  What  is  a  telescope  ?  How  many  kinds  of  telescopes  are  mentioned  ?  What 
ia  the  difference  between  them  1  In.  what  respect  does  the  refracting  telescope  differ 
from  the  compound  microscope  ? 


260  TELESCOPE. 

of  explaining  their  phenomena  for  astronomical  and  nautical 
purposes. 

The  principle  of  the  telescope  will  be  readily  comprehended 
after  what  has  been  said  concerning  the  compound  microscope, 
for  the  two  instruments  differ  chiefly  in  respect  to  the  place  of 
the  object  lens,  that  of  the  microscope  having  a  short,  while 
that  of  the  telescope  has  a  long,  focal  distance. 

84.  REFRACTING  TELESCOPE. — The  most  simple  refracting 
telescope  consists  of  a  tube,  containing  two  convex  lenses,  the 
one  having  a  long,  and  the  other  a  short,  focal  distance.  (The 
focal  distance  of  a  double-convex  lens,  it  will  be  remembered,  is 
nearly  the  center  of  the  sphere,  of  which  it  is  a  part.  56.) 
These  two  lenses  are  placed  in  the  tube,  at  a  distance  from  each 
other  equal  to  the  sums  of  their  two  focal  distances. 

FIG.  220. 


Principle  of  the  Telescope. 

Thus,  if  the  focus  of  the  object  glass,  A,  Fig.  220,  be  eight 
inches,  and  that  of  the  eyeglass  B,  two  inches,  then  the  distance 
of  the  sums  of  the  foci  will  be  ten  inches,  and,  therefore,  the 
two  lenses  must  be  placed  ten  inches  apart ;  and  the  same  rule 
is  observed,  whatever  may  be  the  focal  lengths  of  any  two 
lenses. 

Now,  to  understand  the  "effect  of  this  arrangement,  suppose 
the  rays  of  light,  C  D,  coming  from  a  distant  object,  as  a  star, 
to  fall  on  the  object  glass,  A,  in  parallel  lines,  and  to  be  re- 
fracted by  the  lens  to  a  focus  at  E,  where  the  image  of  the  star 
will  be  represented.  The  image  is  then  magnified  by  the  eye- 
glass B,  and  thus,  in  effect,  is  brought  near  the  eye. 

All  that  is  effected  by  the  telescope,  therefore,  is  to  form  an 
image  of  a  distant  object,  by  means  of  the  object  Jens,  and  then 
to  assist  the  eye  in  viewing  this  image  as  nearly  as  possible  by 
the  eye  lens. 

84.  How  is  the  most  simple  refracting  telescope  formed  ?  Which  is  rhe  object,  and 
•which  the  eye  lens,  in  Fig.  220  1  What  is  the  rule  by  which  the  distance  of  the  two 
glasses  apart  \s  found  1  How  do  the  two  glasses  act,  to  bring  an  object  near  the  eye  ? 
Explain  Fig.  221,  and  show  how  the  object  comes  to  be  inverted  by  the  two  lenses. 


TELESCOPE.  261 

It  is,  however,  necessary  here  to  state,  that  by  the  last  figure, 
the  principle  only  of  the  telescope  is  intended  to  be  explained, 
for  in  the  common  instrument,  with  only  two  glasses  the  image 
appears  to  the  eye  inverted. 

The  reason  of  this  will  be  seen  by  the  next  figure,  where  the 
direction  of  the  rays  of  light  will  show  the  position  of  the  image. 

FIG.  5221. 


Principle  of  the  Telescope. 

Suppose  A,  Fig.  221,  to  be  a  distinct  object,  from  which  pen- 
cils of  rays  flow  from  every  point  toward  the  object  lens  B. 
The  image  of  A,  in  consequence  of  the  refraction  of  the  rays  by 
the  object  lens,  is  inverted  at  C.  which  is  the  focus  of  the  eye- 
glass D,  and  through  which  the  image  is  then  seen,  still  inverted. 

85.  Spyglass. — The  inversion  of  the  object  is  of  little  conse- 
quence when  the  instrument  is  employed  for  astronomical  pur- 
poses, for  since  the  forms  of  the  heavenly  bodies  are  spherical, 
their  positions,  in  this  respect,  do  not  affect  their  general  ap- 
pearance. But  for  terrestrial  purposes,  this  is  manifestly  a  great 
defect,  and  therefore  those  constructed  for  such  purposes,  as 
ship,  or  spyglasses,  have  two  additional  lenses,  by  means  of 
which,  the  images  are  made  to  appear  in  the  same  position  as 
the  objects.  These  are  called  double  telescopes. 

Such  a  telescope  is  represented  at  Fig.  222,  and  consists  of 
an  object  glass  A,  and  three  eyeglasses,  B  C  and  D.  The  eye- 
glasses are  placed  at  equal  distances  from  each  other,  so  that 
the  focus  of  one  may  meet  that  of  the  other,  and  thus  the 
image  formed"  by  the  object  lens,  will  be  transmitted  through 
the  other  three  lenses  to  the  eye.  The  rays  coming  from  the 
object  0,  cross  each  other  at  the  focus  of  the  object  lens,  and 
thus  form  an  inverted  image  at  F.  This  image  being  also  in 
the  focus  of  the  first  eyeglass,  B,  the  rays  having  passed  through 
this  glass  become  parallel,  for  we  have  seen  in  another  place, 

85.  How  is  the  inversion  of  the  object  corrected  ?  Explain  Fig.  222,  and  show  why 
the  two  additional  lenses  make  the  image  of  the  object  erect.  Does  the  addition  of 
these  two  lenses  make  any  difference  with  the  apparent  magnitude  of  the  object  ? 


262 


Refracting  Telescope. 

that  diverging  rays  are  rendered  parallel  by  refraction  through  a 
convex  lens.  The  rays,  therefore,  pass  parallel  to  the  next  lens 
C,  by  which  they  are  made  to  converge,  and  cross  each  other, 
and  thus  the  image  is  inverted,  and  made  to  assume  the  original 
position  of  the  object  O.  Lastly,  this  image,  being  in  the  focus 
of  the  eyeglass  D,  is  seen  in  the  natural  position. 

The  apparent  magnitude  of  the  object  is  not  changed  by  these 
two  additional  glasses,  but  depends,  as  ia  Fig.  220,  on  the  mag- 
nifying power  of  the  eye  and  object  lenses ;  these  two  glasses 
being  added  merely  for  the  purpose  of  making  the  image  ap- 
pear erect. 

86.  REFLECTING  TELESCOPE. — The  common  reflecting  tele- 
scope consists  of  a  large  tube,  containing  two  concave  reflecting 
mirrors,  of  different  sizes,  and  two  eyeglasses.  The  object  is 
first  reflected  from  the  large  mirror  to  the  small  one,  and  from 
the  small  one,  through  the  two  eyeglasses,  where  it  is  then  seen. 

In  comparing  the  advantages  of  the  two  instruments,  it  need 
only  be  stated,  that  the  refracting  telescope  with  a  focal  length 
of  a  thousand  feet,  if  it  could  be  used,  would  not  magnify  dis- 
tinctly more  than  a  thousand  times,  while  a  reflecting  telescope, 
only  eight  or  nine  feet  long,  will  magnify  with  distinctness 
twelve  hundred  times. 

The  principle  and  construction  of  the  reflecting  telescope  will 
be  understood  by  Fig.  223.  Suppose  the  object  0  to  be  at 
such  a  distance,  that  the  rays  of  light  from  it  pass  in  parallel 
lines,  P  P,  to  the  great  reflector,  R  R.  This  reflector  being 
concave,  the  rays  are  converged  by  reflection,  and  cross  each 
other  at  A,  by  which  the  image  is  inverted.  The  rays  then 
pass  to  the  small  mirror,  B,  which  being  also  concave,  they  are 
thrown  back  in  nearly  parallel  lines,  and  having  passed  the 

86.  How  many  lenses  and  mirrors  form  the  reflecting  telescope  ?  What  are  the 
advantages  of  the  reflectins  over  the  refracting  telescope  ?  Explain  Fig.  223.  and 
show  the  course  of  the  rays  from  the  object  to  the  eye.  Why  is  the  small  mirror  in 
this  instrument  made  to  move  by  means  of  a  screw  ? 


TELESCOPB. 
FIG.  223. 


Reflecting  Teletcope. 

aperture  in  the  center  of  the  great  mirror,  fall  on  the  plano- 
convex lens  E.  By  this  lens  they  are  refracted  to  a  focus,  and 
cross  each  other  between  E  and  D,  and  thus  the  image  is  again 
inverted,  and  brought  to  its  original  position,  or  in  the  position 
of  the  object.  The  rays  then  passing  the  second  eyeglass,  form 
the  ima^e  of  the  object  on  the  retina. 

The  large  mirror  in  this  instrument  is  fixed,  but  the  small 
one  moves  backward  and  forward,  by  means  of  a  screw,  so  as  to 
adjust  the  image  to  the  eyes  of  different  persons.  Both  mirrors 
are  made  of  a  composition,  consisting  of  several  metals  melted 
together. 

&7.  One  great  advantage  which  the  reflecting  telescope  pos- 
sesses over  the  refracting,  appears  to  be,  that  it  admits  of  an 
eyeglass  of  shorter  focal  distance,  and,  consequently,  of  greater 
magnifying  power.  The  convex  object  glass  of  the  refracting 
instrument,  does  not  form  a  perfect  image  of  the  object,  since 
some  of  the  rays  are  dispersed,  and  others"  colored  by  refraction. 
This  difficulty  does  not  occur  in  the  reflected  image  from  the 
metallic  mirror  of  the  reflecting  telescope,  and  consequently  it 
may  be  distinctly  seen,  when  more  highly  magnified. 

The  instrument  just  described  is  called  "  Gregory's  telescope," 
because  some  parts  of  the  arrangement  were  invented  by  Dr. 
Gregory. 

88.  fferschers  Telescope. — In  Dr.  Herschel's  grand  telescope, 
the  largest  then  constructed,  the  reflector  was  48  inches  in 
diameter,  and  had  a  focal  distance  of  40  feet.  This  reflector 
was  three  and  a  half  inches  thick,  and  weighed  2000  pounds. 
Now,  since  the  focus  of  a  concave  mirror  is  at  the-  distance  of 
one-half  the  semi-diameter  of  the  sphere,  of  which  it  is  a  sec- 


87.  \Vhat  IP  the  advantage  of  the  reflecting-  telescope  in  respect  to  the  eyeglass' 
88.  What  was  the  focal  distance  and  diameter  of  the  mirror  in  Dr.  Hersche'i's  great 
telescope  1  Where  is  the  largest  Herschers  telescope  now  in  existence  7  What  ia 
the  diameter  and  focal  distance  of  the  reflector  of  this  telescope  ? 


264  TELESCOPE. 

tion,  Dr.  Herschel's  reflector  having  a  focal  distance  of  40  feet, 
formed  a  part  of  a  sphere  of  160  feet  in  diameter. 

This  great  instrument  was  begun  in  1785,  and  finished  four 
years  afterward.  The  frame  by  which  this  wonder  to  all  astron- 
omers was  supported,  having  decayed,  it  was  taken  down  in 
1822,  and  another  of  20  feet  focus,  with  a  reflector  of  18  inches 
in  diameter,  erected  in  its  place,  by  Herschel's  son. 

The  largest  Herschel's  telescope  now  in  existence  is  that  of 
Greenwich  observatory,  in  England.  This  has  a  concave  re- 
flector of  15  inches  in  diameter,  with  a  focal  length  of  25  feet, 
and  was  erected  in  1820. 

89.  LORD  ROSSE'S  TELESCOPE. — Dr.  Herschel's  telescope  was 
the  largest  ever  constructed  until  recently,  when  a  young  no- 
bleman of  fortune  in  Ireland,  Lord  Rosse",  being  led  by  an  in- 
ventive genius,  and  having,  it  appears,  a  degree  of  enterprise 
not  to  be  deterred  by  difficulties,  projected  the  plan  of  building 
a  telescope  of  a  size  and  power  hitherto  unknown  in  the  world. 

The  following  account  of  the  "  Monster  Telescope,"  as  it  has 
been  called,  is  taken  from  that  of  Thomas  Dick,  LL.  D.,  con- 
tained in  his  works. 

It  appears  that  the  possibility  of  casting  a  speculum,  or  re- 
flector for  a  telescope,  of  six  feet  in  diameter,  was  entertained  by 
his  lordship  in  1840,  though  others  considered  such  an  under- 
taking in  the  light  of  a  chimera.  But  the  trial  being  made 
through  the  perseverance  and  large  expenditures  of  the  pro- 
jector, complete  success  crowned  the  experiment,  a  nearly  per- 
fect casting  of  a  speculum  72  inches  in  diameter  being  the  result. 
Thus  the  difficulty  of  constructing  an  instrument  one-third  larger 
than  Herschel's,  was  at  once  surmounted. 

90.  Composition   and    Casting. — The    composition    of  this 
speculum  is  copper  and  tin  united  very  nearly  in  their  atomic 
proportions,  namely  :  copper  1 26.4,  to  tin  58.9  parts.     A  foundry 
was  constructed  expressly  for  this  great  casting,  the  chimney  of 
which  was  18  feet  high,  and  16-£  feet  square  at  the  foundation. 
The  crucibles  for  containing  the  fused  alloy  were  of  cast  iron,  2 
feet  in  diameter,  and  2£  feet  deep.     Iron  baskets,  suspended  by 
cranes,  were  so  contrived  as  to  receive  the  crucibles  and  their 
melted  contents,  and  swing  them  to  the  mold  into  which,  one 
after  the  other,  they  were  poured.     The  mold,  6  feet  in  diam- 
eter and  5-£  inches  deep,  was  arranged  in  an  exact  horizontal 


89.  Who  constructed  the  largest  telescope  in  the  world  ?  What  is  the  size  of  the 
speculum?  90.  What  is  its  composition  ?  How  much  larger  is  this  instrument  than 
Herschel's  •} 


TELESCOPE.  265 

position  by  means  of  spirit  levels.  The  crucibles  were  10  hours 
in  the  furnace  before  the  metal  was  sufficiently  fluid  to  cast. 
The  speculum  weighed  3  tons — lost  one-eighth  of  an  inch  in 
thickness  by  grinding. 

Grinding. — The  grinding  was  conducted  under  Vater,  the 
moving  power  being  a  steam-engine  of  3  horse  power.  The 
grinder  is  of  cast  iron  with  grooves  in  its  face  to  retain  the  emery, 
and  the  two  faces  having  a  mutual  motion,  both  became  per- 
fect, whatever  might  have  been  their  inequalities.  The  polish- 
ing was  done  by  means  of  a  thin  layer  of  pitch  spread  on  the 
grinder,  on  which  rouge  was  smeared  in  the  form  of  paste  with 
water.  This  process  took  six  hours. 

Construction  of  the  Tube. — The  tube  is  56  feet  long,  made 
of  boards  and  hooped  with  iron.  On  the  inside  at  intervals  of 
8  feet,  are  iron  rings  to  support  the  boards.  Its  diameter  is  7 
feet,  the  whole  being  easily  moved  in  any  direction  by  means 
of  pulleys  and  levers,  a  universal  joint  at  the  lower  end  being 
designed  for  this  purpose. 

Wall  of  Support. — At  a  distance  of  12  feet,  on  each  side  of 
the  instrument  is  a  brick  wall,  72  feet  long,  48  high  on  the  out- 
side, and  56  on  the  inside,  ranging  exactly  on  the  mfcridianal 
line.  These  walls  have  rods  of  iron  and  wood  passing  from  one 
to  the  other,  for  the  support  of  the  telescope,  as  it  is  turned  in 
different  directions. 

The  weight  of  the  speculum  and  tube,  including  that  of  the 
bed  on  which  it  is  sustained,  is  about  15  tons. 

This  being  a  reflecting  telescope,  the  observer  stands  in  a 
gallery  at  the  upper  end,  and  looks  into  the  side  of  the  great 
tube,  where  the  observations  are  made  by  means  of  a  reflecting 
surface  of  4,071  square  inches,  while  Herschel's  great  reflector 
had  a  surface  of  only  1,811  square  inches. 

The  cost  of  this  wonder  of  the  age  is  60,000  dollars. 
Description  of  the  Figure. — The  following  description  of  a 
section  of  Lord  Rosse's  telescope,  Fig.  224,  though  not  so  per- 
fect as  could  be  desired,  is  the  best  we  could  obtain.  It  ex- 
Libits  a  view  of  the  inside  of  the  eastern  wall,  with  the  tube, 
and  machinery  by  which  it  is  moved.  A  is  the  mason- work  on 
the  ground ;  B  the  universal  joint,  which  allows  the  tube  to 
turn  in  all  directions ;  C  the  speculum  in  the  tube ;  E  the  eye- 
piece through  which  the  observer  looks;  F  a  pulley  by  which 
the  tube  is  moved  ;  H  a  chain  attached  to  the  pulley,  and  to 
the  side  of  the  tube  ;  I  a  chain  running  to  K,  the  counterpoise; 
L  a  lever  connecting  the  chain  M  with  the  tube ;  Z  another 

12 


Lord  Rosse's  Telescope. 


cliain  which  passes  from  the  upper  part  of  the  tube  over  a  pul- 
ley at  W,  (not  seen,)  and  crosses  to  the  opposite  wall ;  X  a 
railroad  on  which  the  speculum  is  drawn  either  to  or  from  the 
tube.  The  dotted  line  H,  shows  th*e  course  of  the  weight  R,  as 
the  tube  rises  or  falls.  The  tube  is  moved  from  wall  to  wall 
by  a  ratchet  wheel  at  R,  which  is  turned  by  the  lever  O,  on  the 
circle  N,  the  ends  of  which  are  fixed  in  the  two  walls. 

91.  CAMERA  OBSCURA. —  Camera  obscura  strictly  signifies  a 
darkened  chamber,  because  the  room  must  be  darkened,  in  order 
to  observe  its  effects. 

To  witness  the  phenomena  of  this  instrument,  let  a  room  be 
closed  in  every  direction,  so  as  to  exclude  the  light.  Then  from 
an  aperture,  say  of  an  inch  in  diameter,  admit  a  single  beam  of 
light,  and  the  images  of  external  things,  such  as  the  trees  and 
houses,  and  persons  walking  the  streets,  will  be  seen  inverted 
on  the  wall  opp6site  to  where  the  light  is  admitted,  or  on  a 
screen  of  white  paper,  placed  before  the  aperture. 

92.  The  reason  why  the  image  is  inverted  will  be  obvious, 
when  it  is  remembered  that  the  rays  proceeding  from  the  ex- 
tremities of  the  object  must  converge  in  order  to  pass  through 
the  small  aperture ;  and  as  the  rays  of  light  always  proceed  in 


91  \Arliat  dops  camera  obscura  mean?  Describe  the  phenomena  of  the  camera 
pbscura.  92.  Why  is  the  imajre  formed  by  the  camera  obscura  inverted!  How  may 
an  outline  of  the  image  formed  by  the  camera  obscura  be  taken  1  Describe  the  re- 
volving camera  obscura. 


TELESCOPE. 


267 


Principle  of  the  Camera. 


FIG.  226. 


straight  lines,  they  HQ- 

must    cross   each 

other  at  the  point 

of    admission,    as 

explained     under 

the  article  Vision. 

Thus  the  pencil 
A,  Fig.  225,  com- 
ing from  the  up- 
per part  of  the 
tower,  and  pro- 
ceeding straight, 
will  represent  the 

image  of  that  part  at  B,  while  the  lower  part  C,  for  the  same 
reason,  will  be  represented  at  D.  If  a  convex  lens,  with  a  short 
tube,  be  placed  in  the  aperture  through  which  the  light  passes 
into  the  room,  the  images  of  things  will  be  much  more  perfect, 
and  their  colors  more  brilliant. 

This  instrument  is  sometimes 
employed  by  painters,  in  order 
to  obtain  an  exact  delineation 
of  a  landscape,  an  outline  of  the 
image  being  easily  taken  with 
a  pencil,  when  the  image  is 
thrown  on  a  sheet  of  paper. 

There  are  several  modifica- 
tions of  this  machine,  and  among 
them  the  revolving  camera  ob- 
scura  is  the  most  interesting. 

It  consists  of  a  small  house, 
Fly.  226,  with  a  plane  reflector 
A  B,  and  a  double-convex  lens 
C  B,  placed  at  its  top.  The  re- 
flector is  fixed  at  an  angle  of 

45  degrees  with  the  horizon,  so  Camera  obscura. 

as  to  reflect  the  rays  of  light 

perpendicularly  downward,  and  is  made  to  revolve  quite  around, 
in  either  direction,  by  pulling  a  string. 

Now  suppose  the  small  house  to  be  placed  in  the  open  air 
with  the  mirror,  A  B,  turned  toward  the  east,  then  the  rays  of 
light  flowing  from  the  objects  in  that  direction,  will  strike  the 
mirror  in  the  direction  of  the  lines  0,  and  be  reflected  down 
through  the  convex  lens  C  B,  to  the  table  E  E,  where  they  will 


268 


MAGIC    LANTERN. 


form  in  miniature  a  most  perfect  and  beautiful  picture  of  the 
landscape  in  that  direction.  Then,  by  making  the  reflector  re- 
volve, another  portion  of  the  landscape  may  be  seen,  and  thus 
the  objects,  in  all  directions,  can  be  viewed  at  K  without  chang- 
ing the  place  of  the  instrument. 


MAGIC    LANTERN. 


93.  The  Magic  Lantern  is  a  microscope  on  the  same  principle 
as  the  solar  microscope. — But  instead  of  being  used  to  magnify 
natural  objects,  it  is  commonly  employed  for  amusement,  by 
casting  the  shadows  of  small  transparent  paintings  done  on  glass, 
upon  a  screen  placed  at  a  proper  distance. 


FIG.  227. 


Magic  Lantern. 

Let  a  candle  C,  Fig.  227,  be  placed  on  the  inside  of  a  box  or 
tabe,  so  that  its  light  may  pass  through  the  plano-convex  lens 
N,  and  strongly  illuminate  the  object  O.  This  object  is  gen- 
erally a  small  transparent  painting  on  a  slip  of  glass,  which 
slides  through  an  opening  in  the  tube.  In  order  to  show  the 
figures  in  the  erect  position,  these  paintings  are  inverted,  since 
their  shadows  are  again  inverted  by  the  refraction  of  the  convex 
lens  M. 

In  some  of  these  instruments  there  is  a  concave  mirror,  D,  by 
which  the  object  O,  is  more  strongly  illuminated  than  it  would 
be  by  the  lamp  alone.  The  object  is  magnified  by  the  double- 
convex  lens,  M,  which  is  movable  in  the  tube  by  a  screw,  so 


93.  What  is  the  magic  lantern  1    For  what  purpose  is  this  instrument  emplojed  7 
Describe  the  construction  and  effect  of  the  magic  lantern. 


CHROMATICS. 


269 


that  its  focus  can  be  adjusted  to  the  required  distance.  Lastly, 
there  is  a  screen  of  white  cloth,  placed  at  the  proper  distance, 
on  which  the  image  or  shadow  of  the  picture,  is  seen  greatly 
magniti  d. 

The  pictures  being  of  various  colors,  and  so  transparent,  that 
the  light  of  the  lamp  shines  through  them,  the  shadows  arc 
also  of  various  colors,  and  thus  soldiers  and  horsemen  are  repre- 
sented in  their  proper  costume. 

CHROMATICS,    OR    THE    PHILOSOPHY    OF    COLORS. 

94.  We  have  thus  far  considered  light  as  a  simple  body,  and 
have  supposed  that  all  its  parts  were  equally  refracted,  in  its 
passage,  through  the  several  lenses  described.     But  it  will  now 
be  shown  that  light  is  a  compound  body,  and  that  each  of  its 
rays,  which  to  us  appear  white,  is  composed  of  several  colors, 
and  that  each  color  suffers  a  different  degree  of  refraction,  when 
the  rays  of  light  pass  through  a  piece  of  glass,  of  a  certain  shape. 
This  was  a  discovery  of  Sir  Isaac  Newton. 

95.  SOLAR  SPECTRUM. — If  a  ray,  proceeding  from  the  sun, 
be  admitted  into  a  darkened  chamber,  through  an  aperture  in 
the  window  shutter,  and  allowed  to  pass  through  a  triangular 
shaped  piece  of  glass,  called  a  prism,  the  light  will  be  decom- 
posed, and  instead  of  a  spot  of  white,  there  will  be  seen,  on  the 
opposite  wall,  a  most  brilliant  display  of  colors,  including  all 
those  seen  in  the  rainbow. 


FIG. 


M 


1/hUe  <j>-"  C 


Solar  Spectrum. 


Suppose  S,  Fig.  228,  to  be  a  ray  from  the  sun,  admitted 
through  the  window  shutter  A,  in  such  a  direction  as  to  fall  on 

94.  Who  made  the  discovery,  that  l.<tht  is  a  compound  substance  1    95.  In  what 
manner,  and  by  what  means,  is  light  decomposed  1 


270  CHROMATICS. 

the  floor  at  C,  where  it  would  form  a  round,  white  spot.  Now, 
on  interposing  the  prism  P,  the  ray  will  be  refracted,  and  at  the 
same  time  decomposed,  and  will  form  on  the  screen  M  N,  an 
oblong  figure,  containing  seven  colors,  which  will  be  situated  in 
respect  to  each  other,  as  named  on  the  figure. 

It  may  be  observed,  that  of  all  the  colors,  the  red  is  least  re- 
fracted, or  is  thrown  the  smallest  distance  from  the  direction  of 
the  original  sunbeam,  and  that  the  violet  is  most  refracted,  or 
bent  out  of  that  direction. 

This  oblong  image  containing  the  colored  rays,  is  called  the 
solar  or  prismatic  spectrum. 

96.  decomposition  of  White  Light. — That  the  rays  of  the 
sun  are  composed  of  the  several  colors  above  named,  is  suffi- 
ciently evident  by  the  fact,  that  such  a  ray  is  divided  into  these 
several  colors  by  passing  through  the  prism,  but  in  addition  to 
this  proof,  it  is  found  by  experiment,  that  if  these  several  colors 
be  blended  or  mixed  together,  white  will  be  the  result. 

This  may  be  done  by  mixing  together  seven  powders  whose 
colors  represent  the  prismatic  colors,  and  whose  quantities  are 
to  each  other,  as  the  spaces  occupied  by  each  color  in  the  spec- 
trum. When  this  is  done,  it  will  be  found  that  the  resulting 
color  will  be  a  grayish  white.  A  still  more  satisfactory  proof 
that  these  seven  colors  form  white,  when  united,  is  obtained  by 
causing  the  solar  spectrum  to  pass  through  a  lens,  by  which 
they  are  brought  to  a  focus,  when  it  is  found  that  the  focus  will 
be  the  same  color  as  it  would  be  from  the  original  rays  of  the 
sun. 

97.  Other  Means  of  Decomposing  Light. — The  prism  is  not 
the  only  instrument  by  which  light  can  be  decomposed.     A 
soap  bubble  blown  up  in  the  sun  will  display  most  of  the  pris- 
matic colors.     This  is  accounted  for  by  supposing  that  the  sides 
of  the  bubble  vary  in  thickness,  and  that  the  rays  of  light  are 
decomposed  by  these  variations.     The  unequal  surface  of  mother 
of  pearl,  and  many  other  shells,  send  forth  colored  rays  on  the 
same  principle. 

Two  surfaces  of  polished  glass,  when  pressed  together,  will 
also  decompose  the  light.  Rings  of  colored  light  will  be  ob- 


What  are  the  prismatic  colors,  and  how  do  they  succeed  each  other  in  the  spec- 
trum'? Which  color  is  refracted  most  and  which  least?  90.  When  the  several 
prismatic  colors  are  blended,  what  color  is  the  result  ?  When  the  solar  spectrum  is 
made  to  pass  through  a  lens,  what  is  the  color  of  the  focus?  How  do  we  learn  that 
each  colored  ray  has  a  refractive  power  of  its  own  ?  97.  By  what  other  means  be- 
side the  prism,  can  the  rays  of  light  be  decomposed?  How  may  light  be  decomposed 
by  two  pieces  of  glass  7 


CHROMATICS.  271 

served  around  the  point  of  contact  between  the  two  surfaces, 
and  their  number  may  be  increased  or  diminished  by  the  de- 
grees of  pressure.  Two  pieces  of  common  looking-glass,  pressed 
together  with  the  fingers,  will  display  most  of  the  prismatic 
colors. 

98.  A  variety  of  substances,  when  thrown  into  the  form  of 
the  triangular  prism,  will  decompose  the  rays  of  light,  as  well 
as  a  prism  of  glass.     A  very  common  instrument  for  this  pur- 
pose is  made  by  putting  together  three  pieces  of  plate  glass,  in 
form  of  a  prism.     The  ends  may  be  made  of  wood,  and  the 
edges  cemented  with  putty,  so  as  to  make  the  whole  water-tight. 
When  this  is  filled  with  water,  and  held  before  a  sunbeam,  the 
solar  spectrum  will  be  formed,  displaying  the  same  colors,  and 
in  the  same  order,  as  that  above  "described. 

99.  decomposition  of  Li<jh{  by  a  Circle. — On  this  subject,  a 
curious  and  satisfactory  experiment  may  be  made  by  means  of 
a  dark  center,  and  circle,  with 

divisions  between  them,  repre-  FIG.  229. 

senting  the  proportions  of  the 
prismatic  rays,  and  colored  to 
imitate  them.  The  letters, 
Fig.  229,  show  the  different 
colors  of  the  seven  rays,  and 
the  spaces  they  severally  oc- 
cupy. 

100.  Now  if  this  card,  thus 
colored,  be  placed  on  a  spindle 
and  made  to  turn  rapidlv.  the 
seven  colors  will  entirely  dis- 
appear, and  a  dull  white  only 

will  be  presented  to  the  eye,  Recmtipositim  of  Light. 

instead  of  them. 

101.  Explanation. — Any  color  remains  for  an  instant  on  the 
eye  after  it  is  covered,  or  removed,  and  hence    a  fire-brand 
whirled  rapidly,  appears  a  circle  of  fire.     Two  revolving  colors 
will  thus  be  so  blended  as  to  seem  a  medium  between  them ; 
thus  if  all  the  colors  on  the  card  are  covered,  except  the  yellow 
and  red,  they  will  appear  orange.     And  if  all  the  prismatic 
tints  are  blended,  whether  in  the  form  of  powders,  or  propor- 
tionate, colored,  revolving  surfaces,  they  produce  white. 

98.  Of  what  substances  may  prisms  be  formed,  besides  glass  ?  99.  For  what  ar» 
the  divisions  of  the  circle,  Fig.  229,  intended?  What  is  said  to  be  the  effect  when 
this  card  is  revolved  on  a  spindle  ?  100.  What  color  is  said  to  be  produced  when  all 
the  prismatic  tints  are  mixed  or  revolved  7  101.  What  is  the  explanation  1 


272 


RAINBOW. 


THE    RAINBOW. 

102.  The  rainbow  was  a  phenomenon,  for  which  the  ancienU 
were  entirely  unable  to  account ;  but  after  the  discovery  thai 
light  is  a  compound  principle,  and  that  its  colors  may  be  separ- 
ated by  various  substances,  the  solution  of  this  phenomenon 
became  easy. 

Sir  Isaac  Newton,  after  his  great  discovery  of  the  compound 
nature  of  light,  and  the  different  refrangibility  of  the  colored 
rays,  was  able  to  explain  the  rainbow  on  optical  principles. 

103.  If  a  glass  globe  be  suspended  in  a  room,  where  the  rays 
of  the  sun  can  fall  upon  it,  the  light  will  be  decomposed,  or 
separated  into  several  colored  rays,  in  the  same  manner  as  is 
done  by  the  prism.     A  well  defined  spectrum  will  not,  however, 
be  formed  by  the  globe,  because  its  shape  is  such  as  to  disperse 
some  of  the  rays,  and  converge  others ;  but  the  eye,  by  taking 
different  positions  in  respect  to  the  globe,  will  observe  the  va- 
rious prismatic  colors.     Transparent  bodies,  such  as  glass  and 
water,  reflect  the  rays  of  light  from  both  their  surfaces,  but 
chiefly  from  the  second  surface.     That  is,  if  a  plate  of  naked 
glass  be  placed  so  as  to  reflect  the  image  of  the  sun,  or  of  a 
lamp,  to  the  eye,  the  most  distinct  image  will  come  from  the 
second  surface,  or  that  most  distant  from  the  eye.     The  great 
brilliancy  of  the  diamond  is  owing  to  this  cause.     It  will  be  un- 
derstood directly,  how  this  principle  applies  to  the  explanation 
of  the  rainbow. 

104.  How  the  Bow 
is    Formed.  —  Sup- 
pose the  circle  A  B 
C,  Fig.  230,  to  rep- 
resent a  globe,  or  a 
drop  of  rain,  for  each 
drop  of  rain,   as  it 
falls  through  the  air, 
is  a  small  globe  of 
water.  Suppose  also, 
that  the  sun  is  at  S, 
and  the  eye  of  the 
spectator  at  E.  Now, 
it  has  already  been 

102.  What  discovery  preceded  the  explanation  of  the  rainbow?  Who  first  ex- 
planed  the  rainbow  on  optical  principles  .'  103.  Why  does  not  a  glass  globe  form  a 
•well  defined  spectrum!  From  which  surface  do  transparent  bodies  chiefly  reflect 
the  light  1 


How  the  Bow  is  Formed. 


RAINBOW.  273 

stated,  (103,)  that  from  a  single  globe,  the  whole  solar  spectrum 
is  not  seen  in  the  same  position,  but  that  the  different  colors  are 
seen  from  different  places.  Suppose,  then,  that  a  ray  of  light 
from  the  sun  S,  on  entering  the  globe  at  A  is  separated  into  its 
primary  colors,  and  at  the  same  time  the  red  ray,  which  is  the 
least  refrangible,  is  refracted  in  the  line  from  A  to  B.  From 
the  second,  or  inner  surface  of  the  drop,  it  would  be  reflected  to 
C,  the  angle  of  reflection  being  equal  to  the  angle  of  incidence. 
On  passing  out  of  the  drop,  its  refraction  at  C,  would  be  just 
equal  to  the  refraction  of  the  incident  ray  at  A,  and  therefore 
the  red  ray  would  fall  on  the  eye  at  E.  All  the  other  colored 
rays  would  follow  the  same  law,  but  because  the  angles  of  inci- 
dence and  those  of  reflection  are  equal,  and  because  the  colored 
rays  are  separated  from  each  other  by  unequal  refraction,  it  is 
obvious,  that  if  the  red  ray  enters  the  eye  at  E,  none  of  the 
other  colored  rays  could  be  seen  from  the  same  point. 

105.  From  this  it  is  evident,  that  if  the  eye  of  the  spectator 
is  moved  to  another  position,  he  will  not  see  the  red  ray  com- 
ing from  the  same  drop  of  rain,  but  only  the  blue,  and  if  to  an- 
other position,  the  green,  and  so  of  all  the  others.     But  in  a 
shower  of  rain,  there  are  drops  at  all  heights  and  distances,  and 
though  they  perpetually  change  their  places,  in  respect  to  the 
sun  and  the  eye,  as  they  fall,  still  there  will  be  many  which 
will  be  in  such  a  position  as  to  reflect  the  red  rays  to  the  eye, 
and  as  many  more  to  reflect  the  yellow  rays,  and  so  of  all  the 
other  colors. 

106.  This  will  be  made  obvious  by  Fig.  231,  where,  to  avoid 
confusion,  we  will  suppose  that  only  three  drops  of  rain,  and, 
consequently,  only  three  colors,  are  to  be  seen. 

The  numbers  1,  2,  3,  are  the  rays  of  the  sun,  proceeding  to 
the  drops  ABC,  and  from  which  these  rays  are  reflected  to  the 
eye,  making  different  angles  with  the  horizontal  line  H,  be- 
cause one  colored  ray  is  refracted  more  than  another.  Now, 
suppose  the  red  ray  only  reaches  the  eye  from  the  drop  A,  the 
green  from  the  drop  B,  and  the  violet  from  the  drop  C,  then  the 
spectator  would  see  a  minute  rainbow  of  three  colors.  But 
during  a  shower  of  rain,  all  the  drops  which  are  in  the  position 
of  A,  in  respect  to  the  eye,  would  send  forth  red  rays,  and  no 


104.  Explain  Fig.  230,  anrl  show  the  different  refractions,  and  the  reflection  con- 
cerned in  forming  the  rainbow.  In  the  case  supposed,  why  will  only  the  red  ray 
meet  the  eye?  105.  Suppose  a  person  looking  at  the  rainbow  moves  his  eye,  will  he 
see  the  same  colors  from  the  same  drop  of  rain  ?  106.  Explain  Fig.  231,  and  show 
why  \ve  see  different  colors  from  different  drops  of  rain.  Do  several  persons  see  the 
same  rainbow  at  the  same  time  )  Explain  the  reason  of  this. 

12* 


274 


COLORS    OF    OBJECTS. 
FIG.  231. 


Rainbow. 

other,  while  those  in  the  position  of  B,  would  emit  green  rays, 
and  no  other,  and  those  in  the  position  of  C,  violet  rays ;  and 
so  of  all  the  other  prismatic  colors.  Each  circle  of  colors,  of 
which  the  rainbow  is  formed,  is  therefore  composed  of  reflections 
from  a  vast  number  of  different  drops  of  rain,  and  the  reason 
why  these  colors  are  distinct  to  our  senses,  is,  that  we  see  only 
one  color  from  a  single  drop,  with  the  eye  in  the  same  position. 
It  follows,  then,  that  if  we  change  our  position,  while  looking  at 
a  rainbow,  we  still  see  a  bow,  but  not  the  same  as  before,  and 
hence,  if  there  are  many  spectators,  they  will  all  see  a  different 
rainbow,  though  it  appears  to  be  the  same. 

COLORS    OF    OBJECTS. 

10 7.  Color  Depends  on  Absorption  and  Reflection. — It  ap- 
pears that  the  colors  of  all  bodies  depend  on  some  peculiar 
property  of  their  surfaces,  in  consequence  of  which,  they  absorb 
some  of  the  colored  rays,  and  reflect  the  others.  Had  the  sur- 
faces of  all  bodies  the  property  of  reflecting  the  same  ray  only, 
all  nature  would  display  the  monotony  of  a  single  color,  and 
our  senses,  would  never  have  known  the  charms  of  that  variety 
which  we  now  behold. 

107,  On  what  do  the  colors  of  bodies  depend?    Suppose  all  bodies  reflected  the 
same  ray,  what  would  be  the  consequence  in  regard  to  color  ? 


COLORS    OF    OBJECTS.  275 

108.  To  account  for  such  a  variety  of  colors  as  we  see  in  dif- 
ferent bodies,  it  is  supposed  that  all  substances,  when  made 
sufficiently  thin,  are  transparent,  and  consequently,  that  they 
transmit  through  their  surfaces,  or  absorb,  certain  rays  of  light, 
while  other  rays  are  thrown  back,  or  reflected.     Gold,  for  ex- 
ample, may  be  beat  so  thin  as  to  transmit  some  of  the  rays  of 
light,  and  the  same  is  true  of  several  of  the  other  metals,  which 
are  capable  of  being  hammered  into  thin  leaves,     It  is  there- 
fore most  probable,  that  all  the  metals,  could  they  be  made  suf- 
ficiently thin,  would  permit  the  rays  of  light  to  pass  through 
them.     Most,  if  not  quite  all  mineral  substances,  though  in  the 
mass  they  may  seem  quite  opaque,  admit  the  light  through 
their  edges,  when  broken,  and  almost  every  kind  of  wood,  when 
made   no  thinner    than  writing    paper,    becomes    translucent. 
Thus  we  may  safely  conclude,  that  every  substance  with  which 
we  are  acquainted,  will  admit  the  rays  of  light,  when  made  suf- 
ficiently thin. 

109.  Transparent  Substances. — Transparent,  colorless  sub- 
stances, whether  solid  or  fluid,  such  as  glass,  water,  or  mica, 
reflect  and  m  transmit  light  of  the  same  color ;  that  is,  the  light 
seen  through  these  bodies,  and  reflected  from  their  surfaces,  is 
white.     This  is  true  of  all  transparent  substances  under  ordinary 
circumstances  ;  but  if  their  thickness  be  diminished  to  a  certain 
extent,  these  substances  will  both  reflect  and  transmit  colored 
light  of  various  hues,  according  to  their  thickness.     Thus,  the 
thin  plates  of  mica,  which  are  left  on  the  fingers  after  handling 
that  substance  will  reflect  prismatic  rays  of  various  colors. 

110.  From  such  phenomena.  Sir -Isaac  Newton  concluded, 
that  air,  when  below  the  thickness  of  ha/fa  millionth  of  an  inch, 
ceases  to  reflect  light ;  and  also,  that  water,  when  below  the 
thickness  of  three-eighths  of  a  millionth  of  an  inch,  ceases  to  re- 
flect light.     But  that  both  air  and  water,  when  their  thickness 
is  in  a  certain  degree  above  these  limits,  reflect  all  the  colored 
rays  of  the  spectrum. 

111.  From  a  great  variety  of  experiments  on  this  subject,  Sir 
Isaac  Newton  concludes  that  the  transparent  parts  of  bodies, 
according  to  the  sizes  of  their  transparent  pores,  reflect  rays  of 
one  color,  and  transmit  those  of  another,  for  the  same  reason 


108.  How  is  the  variety  of  colors  accounted  for,  by  considering  all  bodies  trans- 
parent 1'  109  What  is  said  of  the  reflection  of  colored  light  by  transparent  sub- 
stances ?  What  substance  is  mentioned,  as  illustrating  this  fact  ?  11U.  What  is  the 
conclusion  of  Sir-Isaac  Newton,  concerning  the  tenuity  at  which  water  and  air  cease 
to  reflect  light  1  111.  What  is  said  of  the  porous  nature  of  the  solid  bodies  7 


276  ASTRONOMY. 

that  thin  plates,  or  minute  particles  of  air,  water,  and  some 
other  substances,  reflect  certain  rays,  and  absorb  or  transmit 
others,  and  that  this  is  the  cause  of  all  their  colors. 


CHAPTER    XII. 

_  ASTRONOMY. 

112.  THIS  term  is  compounded  of  the  Greek  astra,  the  stars, 
and  nomos  a  law ;  and  hence  signifies  the  laws  of  the  celestial 
bodies. 

Astronomy  is  that  science  which  treats  of  the  motions  and 
appearance  of  the  heavenly  bodies  ;  accounts  for  the  phenomena 
which  these  bodies  exhibit  to  us  ;  and  explains  the  laws  by  which 
their  motions,  or  apparent  motions,  are  regulated. 

Astronomy  is  divided  into  Descriptive,  Physical,  and  Prac- 
tical. 

Descriptive  astronomy  demonstrates  the  magnitudes,  distances, 
and  densities  of  the  heavenly  bodies,  and  explains  the  phenom- 
ena dependent  on  their  motions,  such  as  the  change  of  seasons, 
and  the  vicissitudes  of  day  and  night. 

Physical  astronomy  explains  the  theory  of  planetary  motion, 
and  the  laws  by  which  this  motion  is  regulated  and  sustained. 

Practical  astronomy  details  the  description  and  use  of  astro- 
nomical instruments,  and  develops  the  nature  and  application 
of  astronomical  calculations. 

The  heavenly  bodies  are  divided  into  three  distinct  classes,  or 
systems,  namely,  the  solar  system,  consisting  of  the  sun,  moon, 
and  planets  ;  the  system  of  the  fixed  stars  /  and  the  system  of 
the  comets. 

THE    SOLAR    SYSTEM. 

113.  The  Solar  System  consists  of  the  Sun,  and  forty-two 
other  bodies,  including  the  satellites,  which  revolve  around  him 
at  various  distances,  and  in  various  periods  of  time. 

112.  What  is  astronomy?  How  is  astronomy  divided  7  What  does  descriptive 
astronomy  teach  ]  What  is  the  object  of  physical  astronomy?  What  is  practical 
astronomy?  How  are  the  heavenly  bodies  divided?  113.  O'f  what  does  the  solar 
system  consist  ? 


ASTRONOMY. 


277 


These  bodies,  being  perpetually  in  motion,  are  called  planets, 
from  a  Greek  word  signifying  wanderers,  and  they  are  distin- 
guished with  reference  to  their  centers  of  revolution,  into 
primary  and  secondary. 

114.  The  Primary  planets  are  those  which  revolve  around 
the  sun  as  their  proper  center.     These  are  twelve  in  number ; 
that  nearest  the  sun  being  Mercury,  the  others  follow  in  succes- 
sion, thus:  Venus,  Earth,  Mars,   Vesta,   Ceres,   Pallas,  Juno, 
Jupiter,  Saturn,  Herschel  or  Uranus,  and  Neptune. 

115.  The  Secondary  planets  are  those  which  move  round  the 
primaries,  as  these  move  round  the  sun.     Of  these,  there  are 
nineteen,  called  also  moons,  pr  satellites.     These,  as  we  shall 
see,  like  their  primaries,  complete  their  revolutions  at  various 
periods  of  time. 


PRIMARY    PLANETS. 


116.  The  following  tabular  view  of  the  primary  planets  ex- 
hibits their  respective  diameters  ;  their  distances  from  the  sun; 
the  periods  of  their  revolutions  round  the  sun  ;  the  periods  of 
their  revolutions  round  their  axes,  where  this  is  known ;  and 
their  hourly  motion  through  their  several  orbits. 


Names 
of 
the  Planets. 

Diameter 
in  English 
mites. 

Distances 
from  the  Sun  in 
English  miles. 

Revolution 
round 
the  Sun. 

Periods  of  rerolu- 
lion   on   their   own 
axes. 

Hourly 
motion  in 
miles. 

Days. 

Dars.     Hrs.      M. 

Mercury, 

3,22t 

37.000,000 

88 

15          0          5 

110,000 

Venus. 
The  Earth. 

7,6S7 
7.912 

6S.OOO.OOO 
95.000.000 

it! 

0      23      21 
1        0       0 

80.000 
68,000 

Mars, 

4,189 

144,000.000 

687 

1        0      39 

55.000 

Vesta, 

238 

225.000,000 

1.335 

) 

45.000 

Ceres, 

163 

260,  00.000 

1,631 

T>  Unknown. 

41.000 

Pallas, 

80 

266.000.000 

1.680 

s 

41,000 

Juno, 

1.425 

275,000.000 

2,008 

1        3        0 

45.000 

Jupiter, 

89.170 

490.000000 

4.330* 

0        9      56 

36.000- 

Saturn. 

79.042 

9QO.OOO;000 

10.746| 

0      10      16 

22,000 

Herschel, 

3.3.112 

l.SOO.O'O.OOO 

30.fi37i 

070 

15,000 

Neptune, 

35.000 

2,850,000,000 

166  ys. 

Unknown. 

Unk'wn. 

NOTE.  The  above  table,  taken  from  the  last  London  (Prof.  Hoblyn's)  edition  of 
our  Philosophy,  is  believed  to  be  correct,  according  to  the  most  recent  observations. 
It  will  be  seen  that  in  the  descriptions  of  the  planets,  round  numbers  are  generally 
employed,  as  being  more  easily  remembered — also  that  the  periodic  revolutions  ol 
the  planets  are  given  in  years,  days  and  hours,  instead  of  days  only,  as  in  the  table. 

117.  A  Year,  what. — A  year  consists  of  the  time  which  it 
takes  a  planet  to  perform  one  complete  revolution  through  its 

114.  What  are  the  bodies  called,  which  revolve  around  the  Sun  as  a  center1?  115 
What  are  those  planets  called  which  revolve  around  these  primaries  as  a  center? 
116.  In  what  order  are  the  several  planets  situated  in  respect  to  the  Sun?  How  long 
does  it  take  each  planet  to  make  its  revolution  around  the  Sun  ?  117.  What  is  a  year  1 


278 


ASTRONOMY. 


orbit,  or  to  pass  once  around  the  Sun.  Our  Earth  performs 
this  revolution  in  about  365  days,  and  therefore  this  is  the 
period  of  our  year.  Mercury  completes  his  revolution  in  88 
days,  and  therefore  his  year  is  no  longer  than  88  of  our  days. 
But  the  planet  Herschel  is  situated  at  such  a  distance  from  the 
Sun,  that  his  revolution  is  not  completed  in  less  than  about  84 
of  our  years.  The  other  planets  complete  their  revolutions  in 
various  periods  of  time,  between  these ;  so  that  the  time  of  these 
periods  is  generally  in  proportion  to  the  distance  of  each  planet 
from  the  Sun. 

118.  Besides  the  above  enumerated  primary  planets,  our  sys- 
tem contains  nineteen  secondary  planets,  or  moons.     Of  these, 
our  Earth  has  one  moon,  Jupiter  four,  Saturn  eight,  and  Her- 
schel six.     None  of  these  moons,  except  our  own,  and  one  or 
two  of  Saturn's,  can  be  seen  without  a  telescope.     The  seven 
other  planets,  so  far  as  has  been  discovered,  are  entirely  with- 
out moons. 

119.  All  the  planets  move  around  the  Sun  from  west  to  east, 
and  in  the  same  direction  do  the  moons  revolve  around  their 
primaries,  with  the  exception  of  those  of  Herschel,  which  appear 
to  revolve  in  a  contrary  direction. 


NEW    PLANETS   AND    ASTEROIDS. 


120.  The  following  table  contains  the  names  of  the  new 
planets  and  asteroids,  with  the  date,  place  of  discovery,  and  the 
name  of  the  discoverer. 


Name. 

When  discovered. 

By  whom. 

Where. 

Uranus,  . 

March  13,  1781. 

Herschel, 

Slough. 

Ceres,.     .     .     . 

Jan.        1,  1801. 

Piazzi, 

Palermo. 

Pallas,    .     .     . 

March  28,  1802. 

Gibers, 

Bremen. 

Juno,  .... 

Sept.        1,  1804. 

Harding, 

Lilienthral. 

Vesta,     .     .     . 

March  29,  1807. 

Gibers, 

Bremen. 

Astraea,    .     .     . 

Dec.        8,  1845. 

Hencke, 

Driessen. 

Neptune,    . 

Sept.     23,  1846. 

Galle, 

Berlin. 

Hebe,.     .     .     . 

July         1,  1847. 

Hencke, 

Driessen. 

Iris,   .... 

Aug.      13,  1847. 

Hind, 

London. 

Flora,.     .     .     . 

Oct.       18,  1847. 

Hind, 

London. 

Metis,     .     .     . 

.  April    25,  1848. 

Graham, 

Marknee. 

Hygeia,    .     .     . 

April     12,  1849. 

Gasparis, 

Naples. 

Parthenope,     . 

May      13,  1850. 

Gasparis, 

Naples. 

Clio,    .... 

Sept.      13,  1850. 

Hind, 

London. 

Egeria,  .     .     . 

Nov.       2,  1850. 

Gasparis, 

Naples. 

Irene,  .... 

May      20,  1850. 

Hind, 

London. 

New  Planet,    . 

July      29,  1851. 

Gasparis, 

London. 

ASTRONOMY. 

The  preceding  table  is  taken  from  the  American  Almanac 
for  1852. 

With  the  exception  of  Uranus,  or  Herschel,  and  Neptune, 
these  planets  are  called  Asteroids,  meaning  star-like,  or  more 
recently  Planetoids,  planet-like,  on  account  of  their  diminutive 
sizes,  and  in  order  to  distinguish  them  from  the  larger  planets. 

121.  Mr.  Hind  proposed  Victoria,  or  Clio,  for  the  name  of 
the  planet  which  he  discovered  on  the  13th  of  September,  and 
at  first  the  name  of  the  Queen  was  adopted   by  many  foreign 
astronomers.     But  it  seems  that  the  scientific  world  have  .long 
since  refused  to  name  planets  after  their  discoverers,  or  their 
patrons,  or  indeed  after  any  mortal  individual,  choosing  to  adopt 
for  them  the  names  of  heathen  deities,  thus  following  the 
ancient  custom  in  this  respect. 

122.  Number   of  New,    or   Recently   Discovered    Celestial 
Bodies. — In  our  former  edition,  the  solar  system  was  stated  to 
consist  of  the  Sun,  and  twenty-nine  bodies  revolving  around 
him.     At  the  present  time,  the  number  has  increased  to  forty- 
one,  namely,  the  planet  Neptune,  and  eleven  Asteroids,  the 
names  and  dates  of  discovery  of  which,  are  contained  in  the 
preceding  table.     It  has  been  stated  also,  that  an  eighth  satel- 
lite of  Saturn  has  been  discovered,  but  of  this,  we  have  obtained 
no  certain  account. 

The  power  and  perfection  of  new  astronomical  instruments, 
will  probably  lead  to  further  celestial  discoveries,  of  which  the 
world  at  present  can  have  no  conception. 

123.  The  following  table  contains  the  distances  of  the  Aste- 
roids, or  what  recently  have  been  called  the  Planetoids,  from  the 
Sun. 

The  radius  of  the  Earth's  orbit,  in  these  computations,  is  as- 
sumed to  be  95,000,000  of  miles. 

Names.  Distances  from  the  Sun  in  Miles. 

1.  Flora, 209,160,265 

2.  Clio, 221,813,220 

3.  Vesta, 224,302,695 

4.  Iris 226,159,280 

5.  Metis, '.     .  226,632,665 


118.  How  many  moons  does  our  system  contain  7  Which  of  the  planets  are  at- 
tended by  mocns.  and  how  many  has  each  ?  119.  In  what  direction  do  the  planets 
move  around  the  Sun  7  121.  What  is  said  with  respect  to  the  names  of  the  planets? 
1:22.  What  number  of  revolving  celestial  bodies  were  formerly  known  7  How  many 
have  recently  been  discovered,  and  what  are  they  called  1  123.  In  the  above  table 
what  is  the  estimated  distance  of  the  Sun  7 


280  ASTRONOMY. 

Names.  Distances  from  the  Sun  in  Mile*. 

6.  New  Planet, 227,946,800 

7.  Hebe, 230,449,670 

8.  Parthenope, .*    .  232,829,135 

9.  Egeria, 243,206,650 

10.  Irene, 242,468,785 

11.  Astrsea, 244,819,465 

12.  Juno, 253,729,515 

13.  Ceres, 262,964,845 

14.  Pallas, 263,421,510 

15.  Hygeia, 299,255,700 

Observations. — The  periods  of  the  revolutions  of  many  of  the 
recently  discovered  Asteroids  have  not  been  determined.  We 
have,  therefore,  allowed  the  old  ones  to  remain  in  the  table  with 
the  Planets,  as  in  the  former  edition.  It  will  be  observed  that 
there  is  a  difference  between  the  numbers  expressing  the  dis- 
tances of  these  Asteroids  from  the  Sun,  in  the  above,  and  in  the 
former  table.  In  that  the  sums  are  given  in  the  nearest  round 
numbers,  while  in  this,  the  fractions  are  detailed. 

124.  ORBITS  OF  THE  PLANETS. — The  paths  in  which   the 
planets  move  round  the  Sun,  and  in  which  the  moons  move 
round  their  primaries,  are  called  their  orbits.     These  orbits  are 
not  exactly  circular,  as  they  are  commonly  represented  on  pa- 
per, but  are  elliptical,  or  oval,  so  that  all  the  planets  are  nearer 
the  Sun,  when  in  one  part  of  their  orbits  than  when  in  another. 

In  addition  to  their  annual  revolutions,  some  of  the  planets 
are  known  to  have  diurnal,  or  daily  revolutions,  like  our  Earth. 
The  periods  of  these  daily  revolutions  have  been  ascertained,  in 
several  of  the  planets,  by  spots  on  their  surfaces.  But  where 
no  such  mark  is  discernible,  it  can  not  be  ascertained  whether 
the  planet  has  a  daily  revolution  or  not,  though  this  has  been 
found  to  be  the  case  in  every  instance  where  spots  are  seen, 
and,  therefore,  there  is  little  doubt  but  all  have  a  daily  as  well 
as  a  yearly  motion. 

125.  The  axis  of  a  planet  is  an  imaginary  line  passing  through 
its  center,  and  about  which  its  diurnal  revolution  is  performed. 
The  poles  of  the  planets  are  the  extremities  of  this  axis. 

The  orbits  of  Mercury  and  Venus  are  within  that  of  the 

124.  What  i?  the  orbit  of  a  planet?  What  revolutions  have  the  planets,  besides 
their  yearly  revolutions?  Have  all  the  planets  diurnal  revolutions?  How  is  it 
known  that  the  njanets  have  daily  revolutions?  125.  What  is  the  axis  of  a  planet? 
What  is  the  pole  of  a  planet?  Which  are  the  superior,  and  which  the  inferior 
planets  1 


ASTRONOMY. 


281 


Earth,  and  consequently  they  are  called  inferior  planets.  The 
orbits  of  all  the  other  planets  are  without,  or  exterior  to  that  of 
the  Earth,  and  these  are  called  superior  planets. 

126.  That  the  orbits  of  Mercury  and  Venus  are  within  that 
of  the  Earth,  is  evident  from  the  circumstance  that  they  are 
never  seen  in  opposition  to  the  Sun,  that  is,  they  never  appear 
in  the  west  when  the  Sun  is  in  the  east.  '  On  the  contrary,  the 
orbits  of  all  the  other  planets  are  proved  to  be  outside  of  the 
Earth's,  since  these  planets  are  sometimes  seen  in  opposition  to 
the  Sun. 

This  will  be  understood  by  Fig.  232,  where  suppose  S  to  be 
the  Sun,  M  the  orbit  of  Mercury  or  Yenus,  E  the  orbit  of  the 
Earth,  and  J  that  of  Jupiter.  Now,  it  is  evident,  that  if  a 
spectator  be  placed  any  where  on  the  Earth's  orbit,  as  at  E,  he 
may  sometimes  see  Jupiter  in  opposition  to  the  Sun,  as  at  J, 
because  then  the  spectator  would  be  between  Jupiter  and  the 
Sun.  But  the  orbit  of  Venus,  being  surrounded  by  that  of  the 
Earth,  she  never  can  come  in  opposition  to  the  Sun,  or  in  that 
part  of  the  heavens  opposite  to  him,  as  seen  by  us,  because  our 
Earth  never  passes  between  her  and  the  Sun. 

FIG.  232. 


E 
EUiptical  Orbit. 


Orbits  of  the  Planets. 


127.  Orbits  Elliptical. — It  has  already  been  stated,  that  the 
orbits  of  the  planets  are  elliptical,  (124,)  and  that,  consequently, 
these  bodies  are  sometimes  nearer  the  Sun  than  at  others.  An- 


126.  How  is  it  proved  that  the  inferior  planets  are  within  the  Earth's  orbit,  and  the 
superior  ones  without  it?  Explain  Fis.  232,  and  show  why  the  inferior  planets 
never  can  be  in  opposition  to  the  Sun.  127.  What  are  the  shap'es  of  the  planetary  or 
bits  ?  What  is  meant  by  perihelion  7  What  by  aphelion  ? 


282  ASTRONOMY. 

ellipse,  or  oval,  has  two  foci,  and  the  Sun,  instead  of  being  in 
the  common  center,  is  always  in  the  lower  focus  of  their  orbits. 

The  orbit  of  a  planet  is  represented  by  Fig.  233,  where  A  D 
B  E  is  an  ellipse,  with  its  two  foci,  S  and  0,  the  Sun  being  in 
the  focus  S,  which  is  called  the  lower  focus. 

When  the  Earth,  or  any  other  planet,  revolving  around  the 
Sun,  is  in  that  part  of  its  orbit  nearest  the  Sun,  as  at  A,  it  is 
said  to  be  in  its  perihelion  ;  and  when  in  that  which  is  at  the 
greatest  distance  from  the  Sun,  as  at  B,  it  is  said  to  be  in  its 
aphelion.  The  line  S  D,  is  the  mean,  or  average  distance  of  a 
planet's  orbit  from  the  Sun. 

128.  ECLIPTIC. — The  planes  of  the  orbits  of  all  the  planets 
pass  through  the  center  of  the  Sun.     The  plane  of  an  orbit  is 
an  imaginary  surface,  passing  from  one  extremity,  or  side  of  the 
orbit,  to  the  other.     If  the  rim  of  a  drum  head  be  considered  the 
orbit,  its  plane  would  be  the  parchment  extended  across  it,  on 
which  the  drum  is  beaten. 

Let  us  suppose  the  Earth's  orbit  to  be  such  a  plane,  cutting 
the  Sun  through  his  center,  and  extending  out  on  every  side  to 
the  starry  heavens ;  the  great  circle  so  made,  would  mark  the 
line  of  the  ecliptic,  or  the  Sun's  apparent  path  through  the 
heavens. 

The  circle  is  called  the  Sun's  apparent  path,  because  the  rev- 
olution of  the  Earth  gives  the  Sun  the  appearance  of  passing 
through  it.  It  is  called  the  ecliptic,  because  eclipses  happen 
when  the  Moon  is  in,  or  near,  this  apparent  path. 

129.  ZODIAC. —  The  Zodiac  is  an  imaginary  belt,  or  broad 
circle,  extending  quite  around  the  heavens.     The  ecliptic  divides 
the  zodiac  into  two  equal  parts,  the  zodiac  extending  8  degrees 
on  each  side  of  the  ecliptic,  and  therefore  is   16  degrees  wide. 
The  zodiac  is  divided  into  12  equal  parts,  called  the  signs  of  the 
zodiac. 

130.  The  sun  appears  every  year  to  pass  around  the  ^great 
circle  of  the  ecliptic,  and  consequently,  through  the  12  constel- 
lations, or  signs  of  the  zodiac.     But  it  will  be  seen,  in  another 
place,  that  the  Sun,  in  respect  to  the  Earth,  stands  still,  and 
that  his  apparent  yearly  course  through  the  heavens  is  caused 
by  the  annual  revolution  of  the  Earth  around  its  orbit. 

To  understand  the  cause  of  this  deception,  let  us  suppose  that 

128.  What  isthe  plane  of  an  orbit  7  Explnin  what  is  meant  by  the  ecliptic.  Why 
is  the  ecliptic  called  the  Sun's  apparent  path  7  129.  What  is  the  zodiac  ?  How  doea 
the  ecliptic  divide  the  zodiac  7  How  far  does  the  zodiac  extend  on  each  side  of  the 
ecliptic  1  130.  Explain  Fig.  234,  and  show  why  the  Sun  seems  to  pass  through  the 
ecliptic,  when  the  Earth  only  revolves  around  the  Sun  1 


ASTRONOMY. 


283 


S,  Fig.  234,  is  the  Sun,  A  B,  a  part  no.  234. 

of  the  circle  of  the  ecliptic,  and  C 
D,  a  part  of  the  Earth's  orbit.  Now 
if  a  spectator  be  placed  at  C,  he  will 
see  the  Sun  in  that  part  of  the  eclip- 
tic marked  by  B,  but  when  the 
Earth  moves  in  her  annual  revolu- 
tion to  D,  the  spectator  will  see  the 
Sun  in  that  part  of  the  heavens 
marked  by  A ;  so  that  the  motion 
of  the  Earth  in  one  direction,  will 
give  the  Sun  an  apparent  motion  in 
the  contrary  direction. 

131.  CONSTELLATIONS. — A   sign 
or  constellation,  is  a  collection  of 
fixed  stars,  and  as  we  have  already 
seen,   the    Sun    appears   to   move 

through  the  twelve  signs  of  the  zodiac  every  year.  Now,  the 
Sun's  place  in  the  heavens,  or  zodiac,  is  found  by  his  apparent 
conjunction,  or  nearness  to  any  particular  star  in  the  constella- 
tion. Suppose  a  spectator  at  C,  Fig.  234,  observes  the  Sun  to 
be  nearly  in  a  line  with  the  star  at  B,  then  the  Sun  would  be 
near  a  particular  star  in  a  certain  constellation.  When  the 
Earth  moves  to  D,  the  Sun's  place  would  assume  another  direc- 
tion, and  he  would  seem  to  have  moved  into  another  constellation, 
and  near  the  star  A. 

132.  Each  of  the  12  signs  of  the  zodiac  is  divided  into  30 
smaller  parts,  called  degrees ;  each  degree  into  60  equal  parts, 
called  minutes,  and  each  minute  into  60  parts,  called  seconds. 

The  division  of  the  zodiac  into  signs,  is  of  very  ancient  date, 
each  sign  having  also  received  the  name  of  some  animal,  or 
tiling,  which  the  constellation,  forming  that  sign,  was  supposed 
to  resemble.  It  is  hardly  necessary  to  say,  that  this  is  chiefly 
the  result  of  imagination,  since  the  figures  made  by  the  places 
of  the  stars,  never  mark  the  outlines  of  the  figures  of  animals, 
or  other  things.  This  is,  however,  found  to  be  the  most  con- 
venient method  of  finding  any  particular  star  at  this  day,  for 
among  astronomers,  any  star,  in  each  constellation,  may  be  de- 
signated by  describing  the  part  of  the  animal  in  which  it  is 


131.  What  is  a  constellation,  or  sign?  How  is  the  Sun's  apparent  place  in  the 
heavens  found  ?  132.  Into  how  many  parts  are  the  signs  of  the  zodiac  divided,  and 
what  are  these  parts  called?  Is  there  any  resemblance  between  the  places  of  the 
Ftary.  and  the  figures  of  the  animals  after  which  they  are  called  ?  Explain  why  this 
is  a  convenient  method  of  finding  any  particular  star  in  a  sign. 


284 


ASTRONOMY. 


situated.  Thus,  by  knowing  how  many  stars  belong  to  the  con- 
stellation Leo,  or  the  Lion,  we  readily  know  what  star  is  meant 
by  that  which  is  situated  on  the  Lion's  ear  or  tail. 

133.  Names  of  the  Signs. — The  names  of  the  12  signs  of 
the  zodiac  are,  Aries,  Taurus,  Gemini,  Cancer,  Leo,  Virgo,  Libra, 
Scorpio,  Sagittarius,  Capricorn,  Aquarius,  and  Pisces.  The 
common  names,  or  meaning  of  these  words,  in  the  same  order, 
are,  the  Ram,  the  Bull,  the  Twins,  the  Crab,  the  Lion,  the  Vir- 
gin, the  Scales,  the  Scorpion,  the  Archer,  the  Goat,  the  Waterer, 
and  the  Fishes. 

FIG.  235. 


Signs  of  the  Zodiac. 

134.  The  twelve  signs  of  the  zodiac,  together  with  the  Sun, 
and  the  earth  revolving  around  him,  are  represented  at  Fig. 
235.  When  the  Earth  is  at  A,  the  Sun  will  appear  to  be  just 

133.  What  are  the  names  of  the  twelve  signs  7    134.  Explain  why  the  Sun  will  be 
in  the  beginning  of  Aries  when  the  Earth  is  at  A,  Fig.  235. 


ASTRONOMY.  285 

entering  the  sign  Aries,  because  then,  when  seen  from  the  Earth, 
he  ranges  toward  certain  stars  at  the  beginning  of  that  constel- 
lation. When  the  Earth  is  at  C,  the  Sun  will  appear  in  the 
opposite  part  of  the  heavens,  and  therefore  in  the  beginning  of 
Libra.  The  middle  line,  dividing  the  circle  of  the  zodiac  into 
equal  parts,  is  the  line  of  the  ecliptic. 

135.  DENSITY    OF   THE   PLANETS. — Astronomers   have   no 
means  of  ascertaining  whether  the  planets  are  composed  of  the 
same  kind  of  matter  as  our  Earth,  or  whether  their  surfaces  are 
clothed  with  vegetables  and  forests,  or  not.     They  have,  how- 
ever, been  able  to  ascertain  the  densities  of  several  of  them,  by 
observations  on  their  mutual  attraction.     By  density,  is  meant 
compactness,  or  the  quantity  of  matter  in  a  given  space.  (72.) 
When  two  bodies  are  of  equal  bulk,  that  which  weighs  most, 
has  the  greatest  density.     It  was  shown,  while  treating  of  the 
properties  of  bodies,  that  substances  attract  each  other  in  pro- 
portion to  the  quantities  of  matter  they  contain.  (132.)     If, 
therefore,  we  know  the  dimensions  of  several  bodies,  and  can 
ascertain  the  proportion  in  which  they  attract  each  other,  their 
quantities  of  matter,  or  densities,  are  easily  found. 

136.  Thus,  when  the  planets  pass  each  other  in  their  circuits 
through  the  heavens,  they  are  often  drawn  a  little  out  of  the 
lines  of  their  orbits  by  mutual  attraction.     As  bodies  attract  in 
proportion  to  their  quantities  of  matter,  it  is  obvious  that  the 
small  planets,  if  of  the  same  density,  will  suffer  greater  disturb- 
ance from  this  cause,  than  the  large  ones.     But  suppose  two 
planets,  of  the  same  dimensions,  pass  each  other,  and  it  is  found 
that  one  of  them  is  attracted  twice  as  far  out  of  its  orbit  as  the 
other,  then,  by  the  known  laws  of  gravity,  it  would  be  inferred, 
that  one  of  them  contained  twice  the  quantity  of  matter  that 
the  other  did,  and  therefore  that  the  density  of  the  one  was 
twice  that  of  the  other. 

By  calculations  of  this  kind,  it  has  been  found,  that  the 
density  of  the  Sun  is  but  a  little  greater  than  that  of  water,  while 
Mercury  is  more  than  nine  times  as  dense  as  water,  having  a 
specific  gravity  nearly  equal  to  that  of  lead.  The  Earth  has 
a  density  about  five  times  greater  than  that  of  the  Sun,  and  a 
little  less  than  half  that  of  Mercury.  The  densities  of  the  othei 
planets  seem  to  diminish  in  proportion  as  their  distances  from 


135.  How  has  the  density  of  the  planets  been  ascertained  ?  What  is  meant  by  dens 
ty  ?  In  what  proportion  do  bodies  attract  each  other  1  136.  How  are  the  densities 
of  the  planets  ascertained?  What  is  the  density  of  the  Sun,  of  Mercury,  and  of  the 
Earth  ?  la  what  preportions  do  the  densities  of  the  planets  appear  to  diminish  7 


286  THE    SUN. 

the  Sun  increase,  the  density  of  Saturn,  one  of  the  most  remote 
of  planets,  being  only  about  one-third  that  of  water. 


137.  The  Sun  is  the  center  of  the  solar  system,  and  the  grea.t 
dispenser  of  heat  and  light  to  all  the  planets.     Around  the  Sun 
all  the  planets  revolvers  around  a  common  center,  he  being  the 
largest  body  in  our  system,  and,  so  far  as  we  know,  the  largest 
in  the  universe. 

Distance  of  the  Sun. — The  distance  of  the  Sun  from  the 
Earth  is  95  millions  of  miles,  and  his  diameter  is  estimated  at 
887,000  miles.  Our  globe,  when  compared  with  the  magnitude 
of  the  Sun,  is  a  mere  point,  for  his  bulk  is  about  thirteen  hun- 
dred thousand  times  greater  than  that  of  the  Earth.  Were  the 
Sun's  center  placed  in  the  center  of  the  Moon's  orbit,  his  cir- 
cumference would  reach  two  hundred  thousand  miles  beyond 
her  orbit  in  every  direction,  thus  filling  the  whole  space  be- 
tween us  and  the  moon,  and  extending  nearly  as  far  beyond 
her  as  she  is  from  us.  A  traveler,  who  should  go  at  the  rate 
of  90  miles  a  day,  would  perform  a  journey  of  nearly  33,000 
miles  in  a  year,  and  yet  it  would  take  such  a  traveler  more 
than  80  years  to  go  round  the  circumference  of  the  Sun.  A 
body  of  such  mighty  dimensions,  hanging  on  nothing,  it  is  cer- 
tain, must  have  emanated  from  an  Almighty  power. 

The  Sun  appeal's  to  move  around  the  Earth  every  24  hours, 
rising  in  the  east,  and  setting  in  the  west.  This  motion,  as  will 
be  proved  in  another  place,  is  only  apparent,  and  arises  from 
the  diurnal  revolution  of  the  Earth. 

138.  Diurnal  Revolution  of  the  Sun. — The  Sun,  although 
he  does  not,  like  the  planets,  revolve  in  an  orbit,  is,  however, 
not  without  motion,  having  a  revolution  around  his  own  axis, 
once  in  25  days  and  10  hours.     Both  the  fact  that  he  has  such 
a  motion,  and  the  time  in  which  it  is  performed,  have  been  as- 
certained by  the  spots  on  his  surface.     If  a  spot  is  seen,  on  a 
revolving  body,  in  a  certain  direction,  it  is  obvious,  that  when 
the  same  spot  is  again  seen,  in  the  same  direction,  that  the  body 
has  made  one  revolution.     By  such  spots  the  diurnal  revolutions 
of  the  planets,  as  well  as  the  Sun,  have  been  determined. 

137.  Where  is  the  place  of  the  Sun  in  the  solar  system  7  What  is  the  distance  of  th» 
Sun  from  the  Earth  /  What  is  the  diameter  of  the  Sun  ?  Suppo.se  the  center  of  the 
Sun  and  that  of  the  Moon's  orbit  to  be  coincident,  how  far  would  the  Sun  extend  le- 
yond  the  Moon's  orbit  ?  138.  How  is  it  proved  that  the  Sun  has  a  motion  around  hi* 
own  axis?  How  often  does  the  Sun  revolve  7 


MERCURY.  287 

139.  SPOTS  ON  THE  SUN. — Spots  on  the  Sun,  seem  first  to 
have  been  observed  in  the  year  1611,  since  which  time  they 
have  constantly  attracted  attention,  and  have  been  the  subject 
of  investigation  among  astronomers.     These  spots  change  their 
appearance  as  the  Sun  revolves  on  his  axis,  and  become  greater 
or  less^  to  an  observer  on  the  Earth,  as  they  are  turned  to,  or 
from  him ;  they  also  change  in  respect  to  real  magnitude  and 
numl)er ;  one  spot,  seen  by  Dr.  Herschel,  was  estimated  to  t>e 
more  than  six  times  the  size  of  our  Earth,  being  50,000  miles  in 
diameter.     Sometimes  forty  or  fifty  spots  may  be  seen  at  the 
same  time,  and  sometimes  only  one.     They  are  often  so  large 
as  to  be  seen  with  the  naked  eye ;  this  was  the  case  in  1816. 

140.  Nature  and  Design  of  these  Spots. — In  respect  to  the 
nature  and  design  of  these  spots,  almost  every  astronomer  has 
formed  a  different  theory.     Some  have  supposed  them  to  be 
solid  opaque  masses  of  scoriae,  floating  in  the  liquid  fire  of  the 
Sun ;  others,  as  satellites,  revolving  round  him,  and  hiding  his 
light  from  us ;  others,  as  immense  masses,  which  have  fallen  on 
his  disc,  -and  which  are  dark-colored,  because  they  have  not  yet 
become  sufficiently  heated.     From  these  various  theories  we 
may  infer  that,  at  present,  nothing  certain  is  known  of  the  na- 
ture and  design  of  these  spots. 


141.  Mercury,  the  planet  nearest  the  Sun,  is  about  3,000 
miles  in  diameter,  and  revolves  around  him  at  the  distance  of 
37  millions  of  miles.  The  period  of  his  annual  revolution  is  88 
days,  and  he  turns  on  his  axis  once  in  about  15  hours. 

No  signs  of  an  atmosphere  have  been  observed  in  this  planet. 
The  Sun's  heat  at  Mercury  is  about  seven  times  greater  than  it 
is  on  the  Earth,  so  that  water,  if  nature  follows  the  same  laws 
there  that  she  does  here,  can  not  exist  at  Mercury,  except  in  the 
state  of  steam. 

The  nearness  of  this  planet  to  the  Sun,  prevents  his  being 
often  seen.  He  may,  however,  sometimes  be  observed  just  be- 
fore the  rising,  and  a  little  after  the  setting  of  the  Sun.  When 
seen  after  sunset,  he  appears  a  brilliant,  twinkling  star,  showing 

139.  When  were  the  spots  on  the  Sun  first  observed  1  What  has  been  the  differ- 
ence  in  the  number  of  spots  observed  ?  What  was  the  size  of  the  spots  seen  by  Dr. 
Herschel?  110.  What  lias  been  advanced  concerning  the  nature  of  these  spotsl 
H;ive  they  been  accounted  for  satisfactorily  1  141.  What  is  the  diameter  of  Mercury, 
and  what  are  his  periods  of  annual  and  diurnal  revolution  ?  How  great  is  the  Sun's 
heat  at  Mercury?  At  what  times  is  Mercury  to  be  seen?  What  is  a  transit  of 
Mercury  1 


288  VENUS. 

a  white  light,  which,  however,  is  much  obscured  by  the  glare 
of  twilight.  When  seen  in  the  morning,  before  the  rising  of 
the  Sun,  his  light  is  also  obscured  by  the  Sun's  rays. 

Mercury  sometimes  crosses  the  disc  of  the  Sun,  or  comes  be- 
tween the  Earth  and  that  luminary,  so  as  to  appear  like  a  small 
dark  spot  passing  over  the  Sun's  face.  This  is  called  the  transit 
of  Mercury. 


142.  Venus  is  the  other  planet,  whose  orbit  is  within  that  of 
the  Earth.     Her  diameter  is  about  8,000  miles,  being  somewhat 
larger  than  the  Earth. 

Her  revolution  around  the  Sun  is  performed  in  224  days,  at 
the  distance  of  68  millions  of  miles  from  him.  She  turns  on 
her  axis  once  in  23  hours,  so  that  her  day  is  a  little  shorter 
than  ours.  Her  hourly  motion  in  her  orbit,  is  80,000  miles. 

Venus,  as  seen  from  the  Earth,  is  the  most  brilliant  of  all  the 
primary  planets,  and  is  better  known  than  any  nocturnal  lumin- 
arv  except  the  Moon.  When  seen  through  a  telescope,  she  ex- 
hibits the  phases  or  horned  appearance  of  the  moon,  and  her 
face  is  sometimes  variegated  with  dark  spots. 

143.  This  planet  may  often  be  seen  in  the  day  time,  even 
when  she  is  in  the  vicinity  of  the  blazing  light  of  the  Sun.     A 
luminous  appearance  around  this  planet,  seen  at  certain  times, 
proves  that  she  has  an  atmosphere.     Some  of  her  mountains 
are  several  times  more  elevated  than,  any  on  our  globe,  being 
from  10  to  22  miles  high. 

144.  Venus  sometimes  makes  a  transit  across  the  Sun's  disc, 
in  the  same  manner  as  Mercury,  already  described.     The  transits 
of  Venus  occur  only  at  distant  periods  from  each  other.     The 
last  transit  was  in  1769,  and  the  next  will  not  happen  until 
1874.     These  transits  have  been  observed  by  astronomers  with 
the  greatest  care  and  accuracy,  since  it  is  by  observations  on 
them  that  the  true  distances  of  the  Earth  and  planets  from  the 
Sun  are  determined. 

145.  Motions  and  Phenomena  of  Venus. — Sometimes  Venus 
appears  to  recede  from  the  Sun,  and  then  approach  him,  and 
as  her  orbit  is  within  that  of  the  earth,  her  distance  from  us 


142.  Where  is  the  orbit  of  Venus,  in  respect  to  that  of  the  Earth?  What  is  the 
time  of  Venus' revolution  round  the  Sun?  How  often  does  she  turn  on  her  axis? 
143.  What  is  said  of  the  height  of  the  mountains  in  Venus?  144.  On  what  account 
are  the  transits  of  Venus  observed  with  great  care  ?  145.  What  is  the  least  and  great- 
est distance  of  Venus  from  the  Earth  ?  When  is  she  in  her  inferior,  and  when  in  her 
tUferior  conjunction  with  the  Sun  ?  Why  is  she  invisible  in  those  two  positions  7 


V  EXITS. 


289 


FIG.  236. 


varies  from  27,000.000  to  163,000,000  of  miles.  When  near- 
est the  earth  she  forms  her  inferior  conjunction  with  the  Sun ; 
that  is,  she  is  between  us  and  him,  and  hence  being  overpow- 
ered with  his  light,  is  invisible.  When  at  the  greatest  distance 
from  us,  she  forms  her  superior  conjunction  with  that  luminary, 
and  for  the  same  reason  again  becomes  invisible  to  us. 

These  phenomena  will 
be  understood  by  the  fol- 
io wing  explanations  in  con- 
nection with  Fig.  236, 
which  we  quote  from  Dr. 
Dick. 

•146.  Let  the  earth  be 
supposed  at  K,  then  when 
Venus  is  in  the  position 
marked  A,  it  is  in  a  line 
with  the  Sun  as  seen  from 
the  earth,  and  is  then  in 
its  superior  conjunction, 
being  in  the  remotest  part 
of  its  orbit.  When  in  this 
position,  the  whole  of  its 
enlightened  hemisphere  is 
toward  the  earth,  but  is  invisible  on  account  of  the  Sun's  light. 
As  it  moves  from  A  to  B,  being  from  west  to  east,  which  is 
called  its  direct  motion,  it  begins  to  appear  after  sunset  as  the 
evening  star.  When  at  B,  it  appears  among  the  stars  at  L, 
when  it  appears  in  a  gibbous  shape,  nearly  half  its  disc  being( 
luminous.  When  at  C,  it  appears  among  the  stars  at  M,  nearly' 
in  the  form  of  a  half  moon.  At  D,  being  at  the  point  of  its 
greatest  elongation,  it  has  the  form  of  a  half  moon,  and  is  seen 
among  the  stars  at  N.  It  now  appears,  for  some  time  to  be 
stationary,  because  moving  nearly  in  a  straight  line  toward  the 
earth,  its  motion  is  not  seen  ;  when  it  again  appears  to  move 
rapidly,  but  in  a  contrary  direction  from  before,  or,  from  east 
to  west,  during  which  it  presents,  the  form  of  a  crescent.  It 
now  gradually  becomes  so  overpowered  by  the  Sun's  rays  as 
again  to  be  invisible  to  the  naked  eye,  and  when  arrived  at  E, 
forms  her  inferior  conjunction  with  the  Sun,  and  her  nearest 
approach  to  the  Earth.  . 

147.  In  this  position,  Venus  is  36,000,000  miles  nearer  the 


Phases  of  Venus. 


146.  Describe  by  means  of  Fig.  236,  the  phases  of  Venus,  from  her  superior  lo  her 
inferior  conjunction  with  the  Sun. 

13 


290  THE    EARTH. 

Earth  than  when  in  her  superior  conjunction,  and  hence  the 
great  difference  in  her  apparent  size,  and  the  luster  with  which 
she  shines  upon  us.  When  near  her  superior  conjunction, 
almost  her  entire  disc  is  enlightened  to  us,  and  yet  she  appears 
like  a  faint  star  when  compared  with  her  luster  when  near  her 
inferior  conjunction,  and  when  only  her  small  crescent  is  turned 
toward  us. 

Having  passed  her  inferior  conjunction,  her  light  becomes 
less  and  less  until  she  again  becomes  invisible,  as  she  again  ap- 
proaches her  superior  conjunction,  as  before. 

148.  When  Venus  is  in  that  part  of  her  orbit  which  gives 
her  the  appearance  of  being  west  of  the  Sun,  she  rises  before 
him,  and  is  then  called  the  morning  star ;  and  when  she  ap- 
pears east  of  the  Sun,  she  is  behind  him  in  her  course,  and  is 
then  called  the  evening  star.  These  periods  do  not  agree,  either 
with  the  yearly  revolution  of  the  Earth,  or  of  Venus,  for  she  is 
alternately  290  days  the  morning  star,  and  290  days  the  even- 
ing star.  The  reason  of  this  is,  that  the  Earth  and  Venus 
move  round  the  Sun  in  the  same  direction,  and  hence  her  rela- 
tive motion,  in  respect  to  the  Earth,  is  much  slower  than  her 
absolute  motion  in  her  orbit.  If  the  Earth  had  no  yearly  mo- 
tion, Venus  would  be  the  morning  star  one  half  of  the  year, 
and  the  evening  star  the  other  half. 


THE    EARTH. 

149.  The  next  planet  in  our  system,  nearest  the  Sun,  is  the 
t Earth.  Her  diameter  is  8,000  miles.  This  planet  revolves 
around  him  in  365  days,  5  hours,  and  48  minutes ;  and  at  the 
distance  of  95  millions  of  miles.  It  turns  round  its  own  axis 
once  in  24  hours,  making  a  day  and  a  night.  The  Earth's  rev^ 
olution  around  the  Sun  is  called  its  annual  or  yearly  motion, 
because  it  is  performed  in  a  year ;  while  the  revolution  around 
its  own  axis,  is  called  the  diurnal  or  daily  motion,  because  it 
takes  place  every  day.  The  earth's  motion  in  her  orbit  is  at 
the  rate  of  68,000  miles  per  hour.  The  figure  of  the  Earth, 
with  the  phenomena  connected  with  her  motion,  will  be  ex- 
plained in  another  place. 


147.  Why  is  the  light  of  Verms  to  us  so  much  less  at  some  times  than  at  others? 
How  much  nearer  the  e^rth  is  this  planet  at  h^r  inferior  than  at  her  superior  con- 
junction ?  148.  When  is  Venus  the  morning,  and  when  the  evening  star?  How  long 
is  Venus  the  morning  and  how  long  the  evening  star?  149.  How  long  does  it  take 
the  Earth  to  revolve  round  the  Sun  ?  What  is  meant  by  the  Earth's  annual  revolu- 
tion, and  what  by  her  diurnal  revolution  ? 


MARS.  291 


THE    MOON. 

150.  The  Moon,  next  to  the  Sun,  is,  to  us,  the  most  brilliant 
and  interesting  of  all  the  celestial  bodies.  Being  the  nearest  to 
us  of  any  of  the  heavenly  orbs,  and  apparently  designed  for  our 
use,  she  has  been  observed  with  great  attention,  and  many  of 
the  phenomena  which  she  presents,  are  therefore  better  under- 
stood and  explained,  than  those  of  the  other  planets. 

While  the  Earth  revolves  round  the  sun  in  a  year,  it  is  at- 
tended by  the  Moon,  which  makes  a  revolution  round  the  Earth 
once  in  27  days,  7  hours,  and  43  minutes.  The  distance  of 
the  Moon  from  the  Earth  is  240,000  miles,  and  her  diameter 
about  2,000  miles. 

Her  surface,  when  seen  through  a  telescope,  appears  diversi- 
fied with  hills,  mountains,  valleys,  rocks,  and  plains,  presenting 
a  most  interesting  and  curious  aspect :  but  the  explanation  of 
these  phenomena  are  reserved  for  another  section. 


151.  The  next  planet  in  the  solar  system,  is  Mars,  his  orbit 
surrounding  that  of  the  Earth.     The  diameter  of  this  planet  is 
upward  of  4,000  miles,  being  about  half  that  of  the  Earth. 
The  revolution  of  Mars  around  the  Sun  is  performed  in  nearly 
687  days,  or  in  somewhat  less  than  two  of  our  years,  and  he 
turns  on  his  axis  once  in  24  hours  and  40  minutes.     His  mean 
distance  from  the  Sun  is  144,000,000  of  miles,  so  that  he  moves 
in  his  orbit  at  the  rate  of  about  55,000  miles  in  an  hour.     The 
days  and  nights  at  this  planet,  and  the  different  seasons  of  the 
y^ar,  bear  a  considerable  resemblance  to  those  of  the  Earth. 
The  density  of  Mars  is  less  than  that  of  the  Earth,  being  only 
three  times  that  of  water. 

152.  Telescopic  View  of  Mars.  —  This  planet,  to  the  naked 
eye,  reflects  a  yellowish,  or  dull  red  light,  by  which  he  may  be 
distinguished  from  all  the  others.     His  telescopic  appearance  is 
quite  peculiar,  and  often  interesting,  on  account  of  the  changes 
his  face  presents,  being  sometimes  spotted,  then  striped,  then 
clouded,  and  so  on  ;  and  sometimes  all  these  figures  appear  at 


150.  Why  are  the  phenomena  of  the  Moon  better  explained  than  those  of  the  other 
planets?  in  what  time  is  H  revolution  of  the  Moon  about  the  Earth  performed? 
What  is  the  distance  of  the  Moon  from  the  Earth?  151.  What  is  the  d:amf-ter  of 
Mars  i  How  much  lo  -.^cr  is  a  year  at  Mars  than  our  year?  What  is  his=  rate  of  mo- 
tion in  his  orbit  \  152.  What  is  his  appearance  through  the  telescope?  How  is  u 
proved  that  Mars  has  an  atmosphere  of  great  density  ?  Why  does  Mars  sometimes 
appear  to  us  larger  than  at  others  1  How  great  is  the  Sun's  heat  at  Mars  1 


292  JUPITER. 

the  same  time,  presenting  a  great  variety  of  aspects,  some  of 
which  are  represented  by  Fig.  237.  It  is  difficult  to  account 
for  these  appearances,  though  they  are  attributed  to  dense  vapor 
in  the  atmosphere  of  the  planet. 

FIG.  237. 


Telescopic  Phases  of  Mars. 

Mars  has  an  atmosphere  of  great  density  and  extent,  as  i» 
proved  by  the  dim  appearance  of  the  fixed  stars,  when  seen 
through  it.  When  any  of  the  stars  are  seen  nearly  in  a  line 
with  this  planet,  they  give  a  faint,  obscure  light,  and  the  nearer 
they  approach  the  line  of  his  disc,  the  fainter  is  their  light,  until 
the  star  is  entirely  obscured  from  the  sight. 

This  planet  sometimes  appears  much  larger  to  us  than  at 
Dthers,  and  this  is  readily  accounted  for  by  his  greater  or  less 
distance.  At  his  nearest  approach  to  the  Earth,  his  distance  is 
only  50  millions  of  miles,  while  his  greatest  distance  is  240 
millions  of  miles;  making  a  difference  in  his  distance  of  190 
millions  of  miles,  or  the  diameter  of  the  Earth's  orbit. 

The  Sun's  heat  at  this  planet  is  less  than  half  that  which  we 
enjoy. 

To  the  inhabitants  of  Mars,  our  planet  appears  alternately  as 
the  morning  and  evening  star,  as  Venus  does  to  us. 

JUPITER. 

153.  Jupiter  is  89,000  miles  in  diameter,  and  performs  his 
annual  revolution  once  in  about  11  of  our  years,  at  the  distance 
of  490  millions  of  miles  from  the  Sun.  This  is  the  largest 
planet  in  the  solar  system,  being  about  1,400  times  larger  than 

153.  What  is  the  diameter  of  Jupiter  1  What  is  his  distance  from  the  Sun  ?  What 
is  the  period  of  Jupiter's  diurnal  revolution  ?  What  is  the  Sun's  heat  and  light  at 
Jupiter,  when  compared  with  that  of  the  Earth  ?  For  what  is  Jupiter  particularly 
distinguished  1  Is  the  appearance  of  Jupiter's  belts  always  the  same,  or  do  they 
change  ?  What  is  said  of  the  cause  of  Jupiter's  belted  appearance? 


JUPITER.  293 

the  Earth.  His  diurnal  revolution  is  performed  in  nine  hours 
and  titty -six  minutes,  giving  his  surface,  at  the  equator,  a  mo- 
tion of  28,000  miles  per  hour.  This  motion  is  about  twenty 
times  more  rapid  than  that  of  our  Earth  at  the  equator. 

Jupiter,  next  to  Venus,  is  the  most  brilliant  of  the  planets, 
though  the  light  and  heat  of  the  Sun  on  him  is  nearly  25  times 
less  than  on  the  earth. 

This  planet  is  distinguished  from  all  the  others,  by  an  ap«  ^ 
pearance  resembling  bands,  which  extend  across  his  disc.     These/ 
are  termed  belts,  and  are  variable,  both  in  respect  to  number* 
and  appearance.     Sometimes  seven  or  eight  are  seen,  several  of 
which  extend  quite  across  his  face,  while  others  appear  broken, 
or  interrupted. 

FIG.  238. 


Belts  of  Jupiter. 

These  bands,  or  belts,  when  the  planet  is  observed  through  a 
telescope,  appear  as  represented  in  Fig.  238.  This  appearance 
is  much  the  most  common,  the  belts  running  quite  across  the 
face  of  the  planet  in  parallel  lines.  Sometimes,  however,  his 
aspect  is  quite  different  from  this,  for  in  1780,  Dr.  Herscjiel 
saw  the  whole  disc  of  Jupiter  covered  with  small  curved  lines, 
each  of  which  appeared  broken,  or  interrupted,  the  whole  hav- 
ing a  parallel  direction  across  his  disc,  as  in  Fig.  239. 

154.  Jupiter  has  four  satellites,  or  moons,  two  of  which  are 
sometimes  seen  with  the  naked  eye.  They  move  round,  and 
attend  him  in  his  yearly  revolution,  as  the  Moon  does  our  Earth. 
They  complete  their  revolutions  at  different  periods,  the  shortest 
of  which  is  less  than  two  clays,  and  the  longest  seventeen  days. 

154.  How  many  moons  has  Jupiter,  and  what  are  the  periods  of  their  revolutions? 


294 


Occasional  Views  of  Jupiter. 

155.  Eclipses  of  Jupiter* s  Moons. — These  satellites  often  fall 
into  the  shadow  of  their  primary,  in  consequence  of  which  they 
are  eclipsed,  as  seen  from  the  Earth.  The  eclipses  of  Jupiter's 
moons  have  been  observed  with  great  care  by  astronomers,  be- 
cause they  have  been  the  means  of  determining  the  exact  longi- 
tude of  places,  and  the  velocity  with  which  light  moves  through 
space.  How  longitude  is  determined  by  these  eclipses,  can  not 
be  explained  or  understood  at  this  place,  but  the  method  by 
which  they  become  the  means  of  ascertaining  the  velocity  of 
light,  may  be  readily  comprehended.  An  eclipse  of  one  of  these 
satellites  appears,  by  calculation,  to  take  place  sixteen  minutes 
sooner,  when  the  Earth  is  in  that  part  of  her  orbit  nearest  to 
Jupiter,  than  it  does  when  the  Earth  is  in  that  part  of  her  orbit 
at  the  greatest  distance  from  him.  Hence,  light  is  found  to  be 
sixteen  minutes  in  crossing  the  Earth's  orbit,  and  as  the  Sun  is 
in  the  center  of  this  orbit,  or  nearly  so,  it  must  take  about  eight 
minutes  for  the  light  to  come  from  him  to  us.  Light,  there- 
fore, passes  at  the  velocity  of  95  millions  of  miles,  our  distance 
from  the  Sun,  in  about  eight  minutes,  which  is  nearly  200,000 
miles  in  a  second." 


156.    The  planet  Saturn  revolves  round  the  Sun  in  a  period 
of  about  30  of  our  years,  and  at  the  distance  from  him  of  900 


155.  What  occasions  the  eclipses  of  Jupiter's  moons?  Of  what  uses  are  these 
eclipses  to  astronomers  1  How  is  the  velocity  of  light  ascertained  by  the  eclipses  of 
Jupiter's  satellites?  156.  What  is  the  time  of 'Saturn's  periodic  revolution  round  the 
Sun  ?  What  is  his  distance  from  the  Sun  1  What  his  diameter  .'  What  is  the  pe- 
riod of  his  diurnal  revolution'?  How  many  days  make  a  year  at  Saturn?  How 
many  moons  has  Saturn  1 


SATURN.  295 

millions  of  miles.  His  diameter  fs  79,000  miles,  making  his 
bulk  nearly  nine  hundred  times  greater  than  that  of  the  Earth, 
but  notwithstanding  this  vast  size,  he  revolves  on  his  axis  once 
in  about  ten  hours.  Saturn,  therefore,  performs  upward  of 
25,000  diurnal  revolutions  in  one  of  his  years,  and  hence  his 
year  consists  of  more  than  25,000  days ;  a  period  of  time  equal 
to  more  than  10,000  of  our  days.  On  account  of  the  remote 
distance  of  Saturn  from  the  Sun,  he  receives  only  about  a  90th 
part  of  the  heat  and  light  which  we  enjoy  on  the  Earth.  But 
to  compensate,  in  some  degree,  for  this  vast  distance  from  the 
Sun^  Saturn  has  seven  moons,  which  revolve  round  him  at  dif- 
ferent distances,  and  at  various  periods,  from  1  to  80  days. 

157.  Rings  of  Saturn. — Saturn  is  distinguished  from  the 
other  planets  by  his  ring,  as  Jupiter  is  by  his  belt.  When  this 
planet  is  viewed 

through  a  tele-  FIG 

scope,  he  ap- 
pears surround- 
ed by  an  im- 
mense luminous 
circle,  which  is 
represented  by 
Fig.  240. 

There  are  in- 
deed IwO  lumill-  Saturn  and  his  Ring  . 

ous    circles,    or 

rings,  one  within  the  other,  with  a  dark  space  bei  ween  them,  so 
that  they  do  not  appear  to  touch  each  other.  N<  ither  does  the 
inner  ring  touch  the  body  of  the  planet,  there  bei  ng,  by  estima- 
tion, about  the  distance  of  thirty  thousand  miles  between  them. 

The  external  circumference  of  the  outer  ring  is  630,000  miles, 
and  its  breadth  from  the  outer  to  the  inner  circumference,  7,200 
miles,  or  nearly  the  diameter  of  our  Earth.  The  dark  space, 
between  the  two  rings,  or  the  interval  between  the  inner  and 
the  outer  ring,  is  2,800  miles. 

A  third  ring,  interior  to  those  heretofore  known,  was  discov- 
ered in  1851,  by  Mr.  Bond,  of  Cambridge,  Mass. 

This  immense  appendage  revolves  round  the  Sun  with  the 
planet, — performs  daily  revolutions  with  it,  and,  according  to 


157.  How  is  Saturn  particularly  distinguished  from  all  the  other  planets?  What 
distance  is  there  between  the  body  of  Saturn  and  his  inner  rin??  What  distance  is 
there  between  his  inner  and  outer  ring  7  What  is  the  circumference  of  the  outer 

ring"? 


206 


IIERSCHEL. 


FIG.  241. 


Dr.  Ilerschel,  is  a  solid  substance,  equal  in  density  to  the  body 
of  the  planet  itself. 

The  design  of  Saturn's  ring,  an  appendage  so  vast,  and  so 
different  from  any  thing  presented  by  the  other  planets,  has 
always  been  a  matter  of  speculation  and  inquiry  among  astron- 
omers. One  of  its  most  obvious  uses  appears  to  be  that  of  re- 
flecting the  light  of  the  Sun  on  the  body  of  the  planet,  and 
possibly  it  may  reflect  the  heat  also,  so  as  in  some  degree  to 
soften  the  rigor  of  so  inhospitable  a  climate. 

158.  As  this  planet  revolves  around  the  Sun,  one  of  its  sides 
is  illuminated  during  one  half  of  the  year,  and  the  other  side 
during  the  other  half;  so  that,  as  Saturn's  year  is  equal  to 
thirty  of  our  years,  one  of  his  sides  will  be  enlightened  and 
darkened,  alternately,  every  fifteen  years,  as  the  poles  of  our 
Earth  are  alternately  in  thg  light  and  dark  every  year. 

Fig.  241  represents 
Saturn  as  seen  by  an 
eye,  placed  at  right-an- 
gles to  the  plane  of  his 
ring.  When  seen  from 
the  Earth,  his  position  is 
always  oblique,  as  repre- 
sented by  Fig.  240. 

The  inner  white  circle 
represents  the  body  of 
the  planet,  enlightened 
by  the  Sun.  The  dark 
circle  next  to  this,  is  the 
unenlightened  space  be- 
tween the  body  of  the 
planet  and  the  inner 
ring,  being  the  dark  ex- 
panse of  the  heavens  be- 
yond the  planet.  The  two  white  circles  are  the  rings  of  the 
planet,  with  the  dark  space  between  them,  which  also  is  the 
dark  expanse  of  the  heavens. 

The  eighth  satellite  of  this  planet,  was  discovered  by  Mr. 
Bond,  the  discoverer  of  this  third  ring,  as  above  stated. 

HERSCHEL. 

159.  In  consequence  of  some  inequalities  in  the  motions  of 


Direct   View  of  Saturn. 


158   How  long  is  one  of  Saturn's  sides  alternately  in  the  light  and  darkl    In  what 
position  is  Saturn  represented  by  Fig.  210 ) 


.   NEPTUNJE.  297 

Jupiter  and  Saturn,  in  their  orbits,  several  astronomers  had  sus- 
pected that  there  existed  another  planet  beyond  the  orbit  of 
Saturn,  by  whose  attractive  influence  these  irregularities  were 
produced.  This  conjecture  was  confirmed  by  Dr.  Herschel,  in 
1781,  who  in  that  year  discovered  the  planet,  which  is  now 
generally  known  by  the  name  of  its  discoverer,  though  called 
by  him  Georgium  Sidus.  The  orbit  of  Herschel  is  beyond 
that  of  Saturn,  and  at  the  distance  of  1,800  millions  of  miles 
from  the  Sun.  To  the  naked  eye,  this  planet  appears  like  a 
star  of  the  sixth  magnitude,  being,  with  the  exception  of  some 
of  the  comets  and  Neptune,  the  most  remote  body,  so  far  ra 
known  in  the  solar  system. 

160.  Herschel  completes  his  revolution  round  the  Sun  in 
nearly  84  of  our  years,  moving  in  his  orbit  at  the  rate  of  15,000 
miles  in  an  hour.     His  diameter  is  35,000  miles,  so  that  his 
bulk  is  about  eighty  times  that  of  the  Earth.     The  light  and 
heat  of fhe  Sun  at  Herschel,  is  about  360  times  less  than  it  is 
at  the  Earth,  and  yet  it  has  been  found,  by  calculation,  that 
this  light  is  equal  to  248  of  our  full  Moons ;  a  striking  proof  of 
the  inconceivable  quantity  of  light  emitted  by  the  Sun. 

This  planet  has  six  satellites,  which  revolve  round  him  at 
various  distances,  and  in  different  times.  The  periods  of  some 
of  these  have  been  ascertained,  while  those  of  the  others  remain 
unknown. 

NEPTUNE. 

161.  The  discovery  of  this  planet  is  a  signal  instance  of  the 
power  of  mathematical  calculations,  applied  to  the  motions  of 
the  celestial  bodies. 

Astronomers,  for  more  than  half  a  century,  had  observed 
from  various  parts  of  the  world,  certain  secular  perturbations  in 
several  of  the  most  remote  members  of  the  solar  system,  espe- 
cially in  Saturn  and  Uranus. 

These  irregular  motions,  due  to  the  law  of  attraction,  could 
not  be  explained  by  the  influence  of  any  known  body,  circula- 
ting around  the  Sun,  and  hence  the  inference  that  there  existed 
in  that  region,  another  planet  not  yet  seen  by  mortal  eyes. 

162.  Two  young  astronomers,  Adams,  an  Englishman,  and 

159.  What  circumstance  led  to  the  discovery  of  Herschel  ?  In  what  year,  and  by 
whom  was  Herschel  discovered  ?  What  is  the  distance  of  Herscnel  from  the  Sunl 
^160.  In  what  period  is  his  revolution  round  the  Sun  performed  ?  What  is  the  diam- 
*eter  of  Herschel  1  What  is  the  quantity  of  light  and  heat  at  Herschel.  when  com 
p.-ir^d  \v:!h  that  of  the  Earth  1  162.  Hv  whom,  and  in  what  manner  was  Neptune 
dsruvr-.l?  jg* 


298 


NEPTUNE. 


Leverrier,  a  citizen  of  France,  unknown  to  each  other,  pursuing 
this  suggestion,  both  demonstrated,  not  only  the  existence  of 
this  undiscovered  body,  but  showed  within  a  few  degrees  the 
point  in  the  heavens  where  it  would  be  found,  and  where  in 
truth  the  discovery  was  made.  Dr.  Galle,  of  Berlin,  sweeping 
the  heavens  with  his  telescope,  according  to  the  directions  of 
these  demonstrators,  first  saw  the  planet  now  called  Neptune,  on 
the  26th  of  September,  1846.  Still  Leverrier  and  Adams,  by 
the  common  consent  of  astronomers  and  the  scientific  world, 
must  have  the  honor  of  this  discovery,  "so  that  the  discovery 
of  Neptune,  has  happily  crowned  two  heads  with  laurels." 

FIG.  242. 


Herschel 


Relative  Distance  of  the  Planets. 

163.  This  planet  was  first  called  Leverrier,  but  it  seems  that 
astronomers  have  long  since  determined  that  new  ones  shall 

163.  What  is  said  about  calling  new  planets  after  their  discoverers? 


NIPTUNE  299 

not  receive  the  names  of  their  discoverers,  but  of  some  heathen 
divinity,  and  hence  Herschel,  the  name  of  the  discoverer,  has 
been  changed  to  Uranus,  and  Leverrier  into  Neptune. 

164.  Distance  of  Neptune. — It  is  stated  in  the  table  of  the 
planets,  that  the  distance  of  Neptune  from  the  Sun,  is  2,850 
millions  of  millions  of  miles.  On  this  subject,  a  curious  calcu- 
lator says,  "  Had  Adam  and  Eve  started  on  a  railway,  to  go 
from  Neptune  to  the  Sun,  at  the  rate  of  fifty  miles  an  hour, 
they  would  not  yet  have  arrived  there,  for  this  planet,  at  the 
above  rate,  is  more  than  6000  years  from  the  center  of  our  system." 

And  yet  this  orb  was  discovered  by  the  science  of  man. 

RELATIVE  SITUATIONS  OF  THE  PLANETS. — Having  now  given 
a  short  account  of  each  planet  composing  the  solar  system,  the 
relative  situations  of  their  several  orbits,  with  the  exception  of 
those  of  the  Asteroids,  are  shown  by  Fig.  242. 

In  this  figure,  the  orbits  are  marked  by  the  signs  of  each 
planet,  of  which  the  first,  or  that  nearest  the  Sun,  is  Mercury, 
the  next  Venus,  the  third  the  Earth,  the  fourth  Mars ;  then 
come  those  of  the  Asteroids,  then  Jupiter,  then  Saturn,  and 
lastly  Herschel. 

FIG.  243. 


Relative  Sizes  of  the  Planets. 

COMPARATIVE  DIMENSIONS  OF  THE  PLANETS. — The  compara- 
tive dimensions  of  the  planets  are  delineated  at  Fig.  243. 

164.  How  long  would  it  take  to  go  from  our  system  to  Neptune,  at  the  rate  of  fifty 
miles  an  hour  7 


300  MOTIONS    OF    THE    PLANETS. 


MOTIONS    OF    THE    PLANETS. 

It  is  said,  that  when  Sir  Isaac  Newton  was  near  demonstra- 
ting the  great  truth,  that  gravity  is  the  cause  which  keeps  the 
heavenly  bodies  in.  their  orbits,  he  became  so  agitated  with  the 
thoughts  of  the  magnitude  and  consequences  of  this  discovery, 
as  to  be  unable  to  proceed  with  his  demonstrations,  and  desired 
a  friend  to  finish  what  the  intensity  of  his  feelings  would  not 
allow  him  to  complete. 

We  have  seen,  in  a  former  part  of  this  work,  (183,)  that  all 
undisturbed  motion  is  straight  forward,  and  that  a  body  pro- 
jected into  open  space,  would  continue,  perpetually,  to  move  in 
a  right  line,  unless  retarded  or  drawn  out  of  this  course  by  some 
external  cause. 

To  account  for  the  motions  of  the  planets  in  their  orbits,  we 
will  suppose  that  the  Earth,  at  the  time  of  its  creation,  was 
thrown  by  the  hand  of  the  Creator  into  open  space,  the  Sun 
having  been  before  created  and  fixed  in  his  present  place. 

165.  Circular  Motion  of  the  Planets. — Under  Compound 
Motion,  (190,)  it  has  been  shown,  that  when  a  body  is  acted  on 
by  two  forces  perpendicular  to  each  other,  its  motion  will  be  in 
a  diagonal  between  the  direction  of  the  two  forces. 

But  we  will  again  here  sup- 
pose that  a  ball  is  moving  in  the  FIG'  244*        „ 
line  M  X,  Fig.  244,  with  a  given 
force,  and  that  another  force  half 
as  great  should  strike  it  in  the 


direction  of  N,  the  ball  would 
then  describe  the  diagonal  of  a 


-M 


parallelogram,      whose     length- 
would    be  just  equal    to  twice  its  Diagonal  Motion. 

bread  tli,  and  the  line  of  the  ball 

would  be  straight,  because  it  would  obey  the  impulse  and  direc- 
tion of  these  two  forces  only. 

Now  let  A.  Fig.  245,  represent  the  Earth,  and  S  the  Sun  ; 
and  suppose  the  Earth  to  be  moving  forward,  in  the  line  from 
A  to  B,  and  to  have  arrived  at  A,  with  a  velocity  sufficient,  in 
a  given  time,  and  without  olisturbance,  to  have  carried  it  to  B. 
But  at  the  point  A,  the  Sun,  S,  acts  upon  the  Earth  with  his 
attractive  power,  and  with  a  force  which  would  draw  it  to  C, 

165.  Suppose  a  body  to  hp  acted  on  by  two  forces  perpendicular  to  each  other,  in 
what  direction  will  it  move?  Why  does  the  ball.  Fig.  244.  move  in  a  straight  line? 
Why  does  the  Earth.  Fig  245,  move  in  a  curved  line?  Explain  Fig.  245.  and  show 
now  the  two  forces  act  to  produce  a  circular  line  of  motion  1 


MOTIONS    OF    THE    PLANETS. 


301 


Circular  Motion  of  the  Planets. 


in  the  same  space  of  time 
that  it  would  otherwise 
have  gone  to  13.  Then  the 
Earth,  instead  of  passing  to 
B,  in  a  straight  line,,  would 
be  drawn  down  to  D,  the 
diagonal  of  the  parallelo- 
gram, AB  DC.  The  line 
of  direction,  in  Fig.  244, 
is  straight,  because  the 
body  moved,  obeys  only 
the  direction  of  the  two 
forces,  but  it  is  curved  from 
A  to  D,  Fig.  245,  in  con- 
sequence of  the  continued  force  of  the  Sun's  attraction,  which 
produces  a  constant  deviation  from  a  right  line. 

When  the  Earth  arrives  at  D,  still  retaining  its  projectile  or 
centrifugal  force,  its  line  of  direction  would  be  toward  N,  but 
while  it  would  pass  along  to  N  without  disturbance,  the  attract- 
ing force  of  the  Sim  is  again  sufficient  to  bring  it  to  E,  in  a 
straight  line,  so  that,  in  obedience  to  the  two  impulses,  it  again 
describes  the  curve  to  O. 

It  must  be  remembered,  in  order  to  account  for  the  circular 
motions  of  the  planets,  that  the  attractive  force  of  the  Sun  is 
not  exerted  at  once,  or  by  a  single  impulse,  as  is  the  case  with 
the  cross  forces,  producing  a  straight  line,  but  that  this  force  is 
imparted  by  degrees,  and  is  constant.  It  therefore  acts  equally 
on  the  Earth,  in  all  parts  of  the  course  from  A  to  D,  and  from 
D  to  O,  Fig.  245.  From  O,  the  Earth  having  the  same  im- 
pulses as  before,  it  moves  in  the  same  curved  or  circular  direc- 
tion, and  thus  its  motion  is  continued  perpetually. 

166.  The  tendency  of  the  Earth  to  move  forward  in  a  straight 
line,  is  called  the  centrifugal  force,  and  the  attraction  of  the 
Sun,  by  which  it  is  drawn  downward,  or  toward  a  center,  is 
called  its  centripetal  force,  and  it  is  by  these  two  forces  that  the 
planets  are  made  to  perform  their  constant  revolutions  around 
the  Sun,  (197.) 

167.  Elliptical  Orbits. — In   the   above  explanation,  it  has 
been  supposed  that  the  Sun's  attraction,  which  constitutes  the 
Earth's  gravity,  was  at  all  times  equal,  or  that  the  Earth  was 
at  an  equal  distance  from  the.  Sun,  in  all  parts  of  its  orbit. 


166.  What  is  the  projectile  force"  of  the.  Earth  called  ?    What  is  the  attractive  fore* 
of  the  Sun.  which  draws  the  Earth  toward  him,  called? 


302 


MOTIONS    OF    THE    PLANETS, 


FIG.  246. 


But,  as  heretofore  explained,  the  orbits  of  all  the  planets  are 
elliptical,  the  Sun  being  placed  in  the  lower  focus  of  the  ellipse. 
The  Sun's  attraction  is,  therefore,  stronger  in  some  parts  of  their 
orbits  than  in  others,  and  for  this  reason  their  velocities  are 
greater  at  some  periods  of  their  revolutions  .than  at  others. 

The  Earth,  therefore,  in  its  journey  round  the  Sun,  moves  at 
very  unequal  velocities,  sometimes  being  retarded,  and  then 
again  accelerated,  by  the  Sun's  attraction. 

168.  Planets  Pass  Equal  Areas  in  Equal  Times. — It  is  an 
interesting  circumstance,  respecting  the  motions  of  the  planets, 
that  if  the  contents  of  their  orbits  be  divided  into  unequal  tri- 
angles, the  acute  angles  of  which  center  at  the  Sun,  with  the 
line  of  the  orbit  for  their  bases,  the  center  of  the  planet  will 
pass  through  each  of  these  bases  in  equal  times. 

This  will  be  understood  by 
Fig.  246,  the  elliptical  circle 
being  supposed  to  be  the 
Earth's  orbit,  with  the  Sun, 
in  one  of  the  foci. 

Now  the  spaces,  1,  2,  3, 
&c.,  though  of  different  shapes, 
are  of  the  same  dimensions, 
or  contain  the  same  quantity 
of  surface.  The  Earth,  we 
have  already  seen,  in  its  jour- 
ney round  the  Sun,  describes 
an  ellipse,  and  moves  more 
rapidly  in  one  part  of  its  orbit 
than  in  another.  But  what- 
ever may  be  its  actual  ve- 
locity, its  comparative  motion  * 

is  through  equal  areas  in  equal  Elliptical  Orbits. 

times.     Thus  its  center  passes 

from  A  to  B,  and  from  B  to  D,  in  the  same  period  of  time,  and 
so  of  all  the  other  divisions  marked  in  the  figure.  If  the  figure, 
therefore,  be  considered  the  plane  of  the  Earth's  orbit,  divided 
into  12  equal  areas,  answering  to  the  12  months  of  the  year, 
the  Earth  will  pass  through  the  same  areas  in  every  month,  but 
the  spaces  through  which  it  passes  will  be  increased,  during 
every  month,  for  one  half  the  year,  and  diminished,  during 
every  month,  for  the  other  half. 


168.  What  is  meant  by  a  planet's  passing  through  equal  spaces  in  equal  times 7 


THE    EARTH.  303 

169.  Wliy  the  Planets  do  not  Fall  to  the  Sun. — The  reason 
why  the  planets,  when  they  approach  near  the  Sun,  do  not  fall 
to  him,  in  consequence  of  his  increased  attraction,  and  why 
they  do  not  fly  off  into  open  space,  when  they  recede  to  the 
greatest  distance  from  him,  may  be  thus  explained. 

Taking  the  Earth  as  an  example,  we  have  shown  that  when 
in  the  part  of  her  orbit  nearest  the  Sun,  her  velocity  is  greatly 
increased  by  his  attraction,  and  that  consequently  the  Earth's 
centrifugal  force  is  increased  in  proportion. 

170.  Now,  the  velocity  of  the  earth  increases  in  an  inverse 
proportion,  as  its  distance  from  the  Sun  diminishes,  and  in  pro- 
portion to  the  increase  of  velocity  is  its  centrifugal  force  in- 
creased ;  so  that,  in  any  other  part  of  its  orbit,  except  when 
nearest  the  Sun,  this  increase  of  velocity  would  carry  the  Earth 
away  from  its  center  of  attraction.     But  this  increase  of  the 
Earth's  velocity  is  caused  by  it£  near  approach  to  the  Sun,  and 
consequently  the  Sun's  attraction  is  increased,  as  well  as  the 
Earth's  velocity.     In  other  terms,  when  the  centrifugal  force  is 
increased,  the  centripetal  force  is  increased  in  proportion,  and 
thus,  while  the  centrifugal  force  prevents  the  Earth  from  falling 
to  the  Sun,  the  centripetal  force  prevents  it  from  moving  off  in 
a  straight  line. 

THE    EARTH. 

1 71.  Proofs  of  the  Earth's  Diurnal  Revolution. — It  is  almost 
universally  believed,  at  the  present  day,  that  the  apparent  daily 
motion  of  the  heavenly  bodies  from  east  to  west,  is  caused  by 
the  real  motion  of  the  Earth  from  west  to  east,  and  yet  there 
are  comparatively  few  who  have  examined  the   evidence   on 
which  this  belief  is  founded.     For  this  reason,  we  will  here  state 
the  most  obvious,  and  to  a  common  observer,  the  most  convinc- 
ing proofs  of  the  Earth's  revolution.     These  are,  first,  the  in- 
conceivable velocity  of  the  heavenly  bodies,  and  particularly  the 
fixed  stars,  around  the  Earth,  if  she  stands  still.     Second,  the 
fact  that  all  astronomers  of  the  present  age  agree,  that  every 
phenomenon  which  the  heavens  present,  can  be  best  accounted 
for,  by  supposing  the  Earth  to  revolve.     Third,  the  analogy  to 
be  drawn  from  many  of  the  other  planets,  which  are  known 

169.  How  is  it  shown,  that,  if  the  motion  of  a  revolving  body  is  increased,  its  pro- 
iectile  force  is  also  increased  ?  170.  By  what  force  is  the  Earth's  velocity  increased 
as  it  approaches  the  Sun  1  When  the  Earth  is  nearest  the  Sun,  why  does  it.  not  fall 
to  him?  When  the  Earth's  centrifugal  force  is  greatest,  what  prevents  its  flying 
from  the  Sun  ?  171.  What  are  the  most  obvious  and  convincing  proofs  that  the  Earth 
revolves  on  its  axis  1 


304  THE    EARTH. 

to  revolve  on  their  axes ;  and  fourth,  the  different  lengths  of 
days  and  nights  at  the  different  planets,  for  did  the  Sun  revolve 
about  the  solar  system,  the  days  and  nights  at  many  of  the 
planets  must  be  of  similar  lengths. 

172.  The  distance  of  the  Sun  from  the  Earth  being  95  mil- 
lions of  miles,  the  diameter  of  the  Earth's  orbit  is  twice  its  dis- 
tance from   the  Sun,  and,   therefore,    190,000,000    of  miles. 
Now,  the  diameter  of  the  Earth's  orbit,  when  seen  from  the 
nearest  fixed  star,  is  a  mere  point,  and  were  the  orbit  a  solid 
mass  of  dark  matter,  it  could  not  be  seen,  with  such  eyes  as 
ours,  from  such  a  distance.     This  is  known  by  the  fact,  that 
these  stars  appear  no  larger  to  us,  even  when  our  sight  is  assisted 
by  the  best  telescopes,  when  the  Earth  is  in  that  part  of  her 
orbit  nearest  them,  than  when  at  the  greatest  distance,  or  in  the 
opposite  part  of  her  orbit.     The  approach,  therefore,  of  190 
millions  of  miles  toward  the  fixed  stars,  is  so  small  a  part  of 
their  whole  distance  from  us,  that  it  makes  no  perceptible  dif- 
ference in  their  appearance. 

173.  Now,  if  the  Earth  does  not  turn  on  her  axis  once  in  24 
hours,  these  fixed  stars  must  revolve  around  the  Earth  at  this 
amazing  distance  once  in  24  hours.     If  the  Sun  passes  around 
the  Earth  in  24  hours,  he  must  travel  at  the  rate  of  nearly 
400,000  miles  in  a  minute ;  but  the  fixed  stars  are  at  least 
400,000  times  as  far  beyond  the  Sun,  as  the  Sun  is  from  us, 
and,  therefore,  if  they  revolve  around  the  Earth,  must  go  at  the 
rate  of  400,000  times  400,000  miles,  that  is,  at  the  rate  of 
160,000,000,000,  or  160  billions  of  miles  in  a  minute;  a  ve- 
locity of  which  we  can  have  no  more  conception  than  of  infinity 
or  eternity. 

174.  In  respect  to  the  analogy  to  be  drawn  from  the  known 
revolutions  of  the  other  planets,  and  the  different  lengths  of 
days  and  nights  among  them,  it  is  sufficient  to  state,  that  to 
the  inhabitants  of  Jupiter,  the  heavens  appear  to  make  a  revo- 
lution in  about  10  hours,  while  to  those  of  Venus,  they  appear 
to  revolve  once  in  23  hours,  and  to  the  inhabitants  of  the  other 
planets  a  similar  difference  seems  to  take  place,  depending  on 
the  periods  of  their  diurnal  revolutions. 

175.  Now,   there  is  no  more  reason  to  suppose   that  tho 
heavens  revolve  round  us,  than  there  is  to  suppose  that  they  re- 

172.  Were  the  Earth's  orbit  a  solid  mass,  could  it  be  seen  by  us  at  the  distance  ot 
the  fixed  stars?  173.  Suppose  the  Earth  stood  still,  how  fast  must  the  Sun  move  to 
co  round  it  in  24  hours  ?  At  what  rate  must  the  fixed  stars  move  to  go  round  it  in  24 
hours?  174  If  the  heavens  appear  to  revolve  every  10  hours  at  Jupiter,  and  every 
24  hours  at  the  Earth,  hew  can  this  difference  be  accounted  for,  if  they  revolve  at  all  7 


HORIZON.  305 

volve  around  any  of  the  other  planets,  since  the  same  apparent 
revolution  is  common  to  them  all ;  and  as  we  know  that  the 
other  planets,  at  least  many  of  them,  turn  on  their  axes,  and  as 
all  the  phenomena  presented  by  the  Earth,  can  be  accounted 
for  by  such  a  revolution,  it  is  folly  to  conclude  otherwise. 

HORIZON. 

176.  The  horizon  is  distinguished  into  the  sensible  and  ra- 
tional.    The  sensible  horizon^  is  tljat  portion  of  the  surface  of 
the  Earth  which  bounds  our  vision,  or  the  circle  around  us, 
where  the  sky  seems  to  meet  the  Earth.     When  the  Sun  rises, 
he  appears  above  the  sensible  horizon,  and  when  he  sets,  he 
sinks  below  it.     The  rational  horizon  is  an  imaginary  line  pass- 
ing through  the  center  of  the  Earth,  and  dividing  it  into  two 
equal  parts. 

177.  DIRECTION  OF  THE  ECLIPTIC. — The  ecliptic,  (128,)  w*e 
have  already  seen,  is  divided  into  360  equal  parts,  called  de- 
grees.    All  circles,  however  large  or  small,  are  divided  into 
degrees,  minutes,  and  seconds,  in  the  same  manner  as  the 
ecliptic. 

The  axis  of  the  ecliptic  is  an  imaginary  line  passing  through 
its  center  and  perpendicular  to  its  plane.  The  extremities  of 
this  perpendicular  line,  are  called  the  poles  of  the  ecliptic. 

If  the  ecliptic,  or  great  plane  of  the  earth's  orbit,  be  consid- 
ered on  the  horizon,  or  parallel  with  it,  and  the  line  of  the 
Earth's  axis  be  inclined  to  the  axis  of  this  'plane,  or  the  axis  of 
the  ecliptic,  at  an  angle  of  2  3  £  degrees,  it  will  represent  the 
relative  positions  of  the  orbit,  and  the  axis  of  the  Earth. 

These  positions  are,  however,  merely  relative,  for  if  the  posi- 
tion of  the  Earth's  axis  be  represented  perpendicular  to  the 
equator,  then  the  ecliptic  will  cross  this  plane  obliquely.  But 
when  the  Earth's  orbit  is  considered  as  having  no  inclination, 
its  axis  of  course  will  have  an  inclination  to  the  axis  of  the 
ecliptic,  of  23-£  degrees. 

As  the  orbits  of  all  the  other  planets  are  inclined  to  the 
ecliptic,  perhaps  it  is  the  most  natural  and  convenient  method 
to  consider  this  as  a  horizontal  plane,  with  the  equator  inclined 

175.  Is  there  any  more  reason  to  believe  that  the  Sun  revolves  round  the  Earth  than 
round  any  of  'the  other  planets  ?  How  can  all  the  phenomena  of  the  heavens  be  ac- 
counted for  if  the  planets  do  not  revolve?  176  How  is  the  sensible  horizon  distin- 
guished from  the  rational?  177.  How  are  circles  divided?  VVnat  is  the  axis  of  the 
ecliptic  ?  What  are  the  poles  of  the  ecliptic  1  How  manv  degrees  is  the  a^is  of  the 
Earth  inclined  to  that  of  the  ecliptic  ?  What  is  said  concerning  the  relative  position* 
of  the  Earth's  aiis  and  the  plane  of  the  ecliptie  ? 


306  HORIZON. 

'  to  it,  instead  of  considering  the  equator  on  the  plane  of  the 
horizon,  as  is  sometimes  done. 

178.  INCLINATION  OF  THE  EARTH'S  Axis. — The  inclination 
of  the  Earth's  axis  to  the  axis  of  its  orbit  never  varies,  but 
always  makes  an  angle  with  it  of  23i  degrees,  as  it. moves 
round  the  Sun.  The  axis  of  the  Earth  is  therefore  always  par- 
allel with  itself.  That  is,  if  a  line  be  drawn  through  the  center 
of  the  Earth,  in  the  direction  of  its  axis,  and  extended  north  and 
south,  beyond  the  Earth's  diameter,  the  line  so  produced  will 
always  be. parallel  to  the  same  line,  or  any  number  of  lines,  so 
drawn,  when  the  Earth  is  in  different  parts  of  its  orbit. 

Suppose  a  rod  to  be  fixed  into  the  flat  surface  of  a  table,  and 
so  inclined  as  to  make  an  angle  with  a  perpendicular  from  the 
table  of  23£  degrees.  Let  this  rod  represent  the  axis  of-  the 
Earth,  and  the  surface  of  the  table,  the  ecliptic.  Now  place  on 
th^e  table  a  lamp,  and  round  the  lamp  hold  a  wire  circle  three 
or  four  feet  in  diameter,  so  that  it  shall  be  parallel  with  the 
plane  of  the  table,  and  as  high  above  it  as  the  flame  of  the 
lamp.  Having  prepared  a  small  terrestrial  globe,  by  passing  a 
wire  through  it  for  an  axis,  and  letting  it  project  a  few  inches 


Inclination  of  the  Earth's  Axis. 


.  178.  Are  the  orbits  of  the  other  planets  parallel  to  the  Earth's  orbit,  or  inclined  to 
it?    What  is  meant  by  the  Earth's  axis  being  parallel  to  itself? 


DAY    AND    NIGHT.  307 

each  way,  for  the  poles,  take  hold  of  the  north  pole,  and  carry 
it  round  the  circle  with  the  poles  constantly  parallel  to  the  rod 
rising  above  the  table.  -The  rod  being  inclined  23-£  degrees 
from  a  perpendicular,  the  poles  and  axis  will  be  inclined  in  the 
same  decree,  and  thus  the  axis  of  the  earth  will  be  inclined  to 
that  of  the  ecliptic  every  where  in  the  same  degree,  and  lines 
drawn  in  the  direction  of  the  Earth's  axis  will  be  parallel  to 
each  other  in  any  part  of  its  orbit. 

179.  This  will  be  understood  by  Fig.  247,  where  it  will  be 
seen,  that  the  poles  of  the  Earth,  in  the  several  positions  of  A 
B  C  and  D,  being  equally  inclined,  are  parallel  to  each  other. 
Supposing  the  lamp  to  represent  the  Sun,  and  the  wire  circle 
the  Earth's  orbit,  the  actual  position  of  the  Earth,  during  its 
annual  revolution  around  the  Sun,  will  be  comprehended,  and 
if  the  globe  be  turned  on  its  axis,  while  passing  round  the  lamp, 
the  diurnal,  or  daily  revolutions  of  the  Earth  will  also  be 
represented. 


DAY   AND    NIGHT. 


180.  Were  the  direction  of  the  Earth's  axis  perpendicular  to 
the  plane  of  its  orbit,  the  days  and  nights  would  be  of  equal 
length  all  the  year,  for  then  just  one  half  of  the  Earth,  from 
pole  to  pole,  would  be  enlightened,  and  at  the  same  time  the 
other  half  would  be  in  darkness. 


FIG. 


Day  and  Night. 

Suppose  the  line  S  o,  Fig.  248,  from  the  Sun  to  the  Earth, 
to  be  the  plane  of  the  Earth's  orbit,  and  that  N  S  is  the  axis  of 
the  Earth  perpendicular  to  it,  then  it  is  obvious,  that  exactly 


179.  How  does  it  appear  by  Fr«r.  247,  that  the  axis  of  the  Earth  is  parallel  to  itself, 
in  ail  parts  of  its  orb;t  ?  ISO.  How  are  the  annual  and  diurnal  revolurons  of  the 
Earth  illustrated  by  Fig  248  7  Explain,  by  Fg.  248,  why  the  days  and  nights  would 
every  where  be  equal,  were  the  axis  of  the  Earth  perpendicular  to  the  plane  of  his 
orbit. 


308  SEASONS    OF    THE    YEAK. 

the  same  points  on  the  Earth  would  constantly  pass  through 
the  alternate  vicissitudes  of  day  and  night;  for  all  who  live  on 
the  meridian  line  between  N  and  S,  which  line  crosses  the 
equator  at  o,  would  see  the  Sun  at  the  same  time,  and  conse- 
quently, as  the  Earth  revolves,  would  pass  into  the  dark  hem- 
isphere at  the  same  time.  Hence,  in  all  parts  of  the  globe,  the 
days  and  nights  would  be  of  equal  length,  at  any  given  place. 

181.  Now  it  is  the  inclination  of  the  Earth's  axis,  as  above 
described,  which  causes  the  lengths  of  the  days  and  nights  to 
differ  at  the  same  place  at  different  seasons  of  the  year ;  for  on 
reviewing  the  position  of  the  globe  at  A,  Fig.  247,  it  will  be 
observed  that  the  line  formed  by  the  enlightened  and  dark 
hemispheres,  does  not  cojncide  with  the  line  of  the  axis  and 
pole,  as  in  Fig.  248,  but  that  the  line  formed  by  the  darkness 
arid  the  light,  extends  obliquely  across  the  line  of  the  Earth's 
axis,  so  that  the  north  pole  is  in  the  light  while  the  south  is  in 
the  dark.     In  the  position  A,  therefore,  an  observer  at  the  north 
pole  would  see  the  sun  constantly,  while  another  at  the  south 
pole  would  not  see  it  at  all.     Hence  those  living  in  the  north 
temperate  zone,  at  the  season  of  the  year  when  the  earth  is  at 
A,  or  in  the  Summer,  would  have  long  days  and  short  nights, 
in  proportion  as  they  approached  the  polar  circle ;  while  those 
who  live  in  the  south  temperate  zone,  at  the  same  time,  and 
when  it  would  be  Winter  there,  would  have  long  nights  and 
short  days  in  the  same  proportion. 

SEASONS    OF    THE    YEAR. 

182.  The  vicissitudes  of  the  seasons  are  caused  by  the  annual 
revolution  of  the  Earth  round  the  Sun,  together  with  the  in- 
clination of  its  axis  to  the.pla.ne  of  its  orbit. 

It  has  already  been  explained,  that  the  ecliptic  is  the  plane 
of  the  Earth's  orbit,  and  is  supposed  to  be  placed  on  a  level 
with  the  Earth's  horizon,  and  hence,  that  this  plane  is  consid- 
ered the  standard,  by  which  the  inclination  of  the  lines  crossing 
the  Earth,  and  the  obliquity  of  the  orbits  of  the  other  planets, 
are  to  be  estimated. 

183.  The  Solstices. — The  solstices  are  the  points  where  the 
ecliptic  and  the  equator  are  at  the  greatest  distance  from  each 

181.  What  is  the  cause  of'the  unequal  lengths  of  the  days  and  nights  in  different 
parts  of  the  world  ?  182  What  are  the  causes  which  produce  the  seasons  of  the 
year?  183.  What  are  the  solstices?  When  the  Sun  enters  the  Summer  solstice, 
what  is  said  of  the  length  of  the  days  and  nights?  When  does  the  Sun  enter  the 
Winter  solstice,  and  what  is  the  proportion  between  the  length  of  the  days  and 
nights  1 


REVOLUTIONS    OF   THE    EARTH. 


809 


other.  The  Earth,  in  its  yearly  revolution,  passes  through  each 
of  these  points.  One  is  called  the  Summer,  and  the  other  the 
Winter  solstice.  The  Sun  is  said  to  enter  the  Summer  solstice 
on  the  21st  of  June;  and  at  this  time,  in  our  hemisphere,  the 
days  are  longest  and  the  nights  shortest.  On  the  21st  of  De- 
cember, he  enters  his  Winter  solstice,  when  the  length  of  the 
days  and  nights  are  reversed  from  what  they  were  in  June  be- 
fore, the  days  being  shortest,  and  the  nights  longest. 

Having  learned  these  explanations,  the  student  will  be  able 
to  understand  in  what  order  the  seasons  succeed  each  other, 
and  the  reason  why  such  changes  are  the  effect  of  the  Earth's 
revolution. 

REVOLUTIONS    OF    THE    EARTH. 

184.  Suppose  the  Earth,  Fig.  24P,  to  be  in  her  Summer 
solstice,  which  takes  place  on  the  21st  of  June.  At  this  period 
4he  will  be  at  A,  having  her  north  pole,  N  so  inclined  toward 

FIG.  249. 


Seasons  of  the  Tear. 

the  Sun,  that  the  whole  arctic  circle  will  be  illuminated,  and 
consequently  the  Sun's  rays  will  extend  23^-  degrees,  the  breadth 
of  the  polar  circle,  beyond  the  north  pole.  The  diurnal  revolu- 
tion, therefore,  when  the  Earth  is  at  A,  causes  no  succession  of 
day  and  night  at  the  pole,  since  the  whole  frigid  zone  is  within 


184.  At  what  season  of  the  year  ia  the  whole  arctic  circle  illuminated  ? 


310  REVOLUTIONS  OF  THE  EARTH. 

the  reach  of  his  rays.     The  people  who  live  within  the  arctic 
circle  will,  consequently,  at  this  time,  enjoy  perpetual  day. 

185.  During  this  period,  just  the  same  proportion  of  tho 
earth  that  is  enlightened  in  the  northern  hemisphere,  will  be  in 
total  darkness  in  the  opposite  region  of  the  southern  hemisphere ; 
so  that  while  the  people  of  the  north  are  blessed  with  perpetual 
day,  those  of  the  south  are  groping  in  perpetual  night.     Those 
who  live  near  the  arctic  circle  in  the  north  temperate  zone,  will, 
during  the  Summer,  come,  for  a  few  hours,  within  the  regions 
of  night,  by  the  Earth's  diurnal  revolution  ;  and  the  greater  the 
distance  from  the.  circle,  the  longer  will  be  their  nights,  and  the 
shorter  their  days. 

186.  Hence,  at  this  season,  the  days  will  be  longer  than  the 
nights  every  where  between  the  equator  and  the  arctic  circle. 
At  the  equator,  the  days  and  nights  will  be  equal,  and  between 
the  equator  and  the  south  polar  circle,  the  nights  will  be  longer 
than  the  days,  in  the  same  proportion  as  the  days  are  longer 
than  the  nights,  from  the  equator  to  the  arctic  circle. 

187.  The  Sun  always  Shines  on  180  Degrees  of  the  Earth. — 
It  will  be  observed  by  a  careful  perusal  of  the  above  explana- 
tion of  the  seasons,  and  a  close  inspection  of  the  figure  by  which 
it  is  illustrated,  that  the  Sun  constantly  shines  on  a  portion  of 
the  Earth  equal  to  90  degrees  north,  and  90  degrees  south, 
from  his  place  in  the  heavens,  and  consequently,  that  he  always 
enlightens  1 80  degrees,  or  one  half  of  the  Earth.     If,  therefore, 
the  axis  of  the  Earth  were  perpendicular  to  the  plane  of  its 
orbit,  the  days  and  nights  would  every  where  be  equal,  for  as 
the  Earth  performs  its  diurnal  revolutions,  there  would  be  12 
hours  day,  and  12  hours  night.     But  since  the  inclination  of  its 
axis  is  23£  degrees,  the  light  of  the  Sun  is  thrown  23£  degrees 
farther  in  that  direction,  when  the  north  pole  is  turned  toward 
the  Sun,  than  it  would,  had  the  Earth's  axis  no  inclination. 
Now,  as  the  Sun's  light  reaches  only  90  degrees  north  or  south 
of  his  place  in  the  heavens,  so  when  the  arctic  circle  is  enlight- 
ened, the  antarctic  circle  must  be  in  the  dark ;  for  if  the  light 
reaches  23£  degrees  beyond  the  north  pole,  it  must  fall  23£  de- 
grees short  of  the  south  pole. 

188.  As  the  Earth  travels  round  the  Sun,  in  his  yearly  cir- 
cuit, this  inclination  of  the  poles  is  alternately  toward  and  from 

185.  At  what  season  is  the  whole  antarctic  circle  in  the  rlark  ?  While  the  people 
near  the  north  pole  enjoy  perpetual  day,  what  is  the  situation  of  those  near  the  south 
pole  ?  186.  At  what  season  will  the  days  be  longer  than  the  niglits  every  where  be- 
tween the  equator  and  the  arctic  circle?  187.  How  many  degrees  does  the  Sun's 
light  reach,  north  and  south  of  him,  on  the  Earth? 


REVOLUTIONS    OF    THE    EARTH.  311 

him.  During  our  "Winter,  the  north  polar  region  is  thrown  be- 
yond the  rays  of  the  Sun,  while  a  corresponding  portion  around 
the  south  pole  enjoys  the  Sun's  light.  And  thus,  at  the  poles, 
there  are  alternately  six  months  of  darkness  and  Winter,  and 
six  months  of  sunshine  and  Summer. 

189.  While  we,  in  the  northern  hemisphere,  are  chilled  by 
the  cold  blasts  of  Winter,  the  inhabitants  of  the  southern  hem- 
isphere are  enjoying  all  the  delights  of  Summer;  and  while 
we  are  scorched  by  the  rays  of  a  vertical  Sun  in  June  and  July, 
our  southern  neighbors  are  shivering  with  the  rigors  of  mid- 
Winter. 

190.  At  the  equator,  no  such  changes  take  place.     The  rays 
of  the  Sun,  as  the  Earth  passes  round  him,  are  vertical  twice  a 
year  at  every  place  between  the  tropics.     Hence,  at  the  equator, 
there  are  two  Summers  and  no  Winter,  and  as  the  Sun  there 
constantly  shines  on  the  same  half  of  the  Earth  in  succession, 
the  days  and  nights  are  always  equal,  there  being  12  hours  of 
light  and  12  of  darkness. 

191.  VELOCITY  OF  THE  EARTH. — The  motion  of  the  Earth 
round  the  Sun,  is  at  the  rate  of  68,000  miles  in  an  hour,  while 
its  motion  on  its  own  axis,  at  the  equator,  is  at  the  rate  of  about 
1,042  miles  in  the  hour.     The  equator  being  that  part  of  the 
Earth  most  distant  from  its  axis,  the  motion  there  is  more  rapid 
than  toward  the  poles,  in  proportion  to  its  greater  distance  from 
the  axis  of  motion. 

192.  The  method  of  ascertaining  the  velocity  of  the  Earth's 
motion,  both  in  its  orbit  and  round  its  axis,  is  simple  and  easily 
understood ;  for  by  knowing  the  diameter  of  the  Earth's  orbit, 
its  circumference  is  readily  found,  and  as  we  know  how  long  it 
takes  the  Earth  to  perform  her  yearly  circuit,  we  have  only  to 
calculate  what  part  of  her  journey  she  goes  through  in  an  hour. 
By  the  same  principle,  the  hourly  rotation  of  the  Earth  is  as 
readily  ascertained. 

193.  We  are  insensible  to  these  motions,  because  not  only 
the  Earth,  but  the  atmosphere,  and  all  terrestrial  things,  partake 
of  the  same  motion,  and  there  is  no  change  in  the  relation  of 
objects  in  consequence  of  it. 


1S8.  Purin?  our  Winter,  is  the  north  pole  turned  to  or-  from  the  Sun  ?  At  th* 
poles,  how  many  days  and  nights  are  there  in  the  year?  189.  When  it  is  Winter  in 
the  northern  hemisphere,  what  is  the  season  in  the  southern  hemisphere  ?  190.  At 
what  rate  does  the  Earth  move  around  the  Pun  ?  What  are  the  seasons  at  ihe  equa- 
tor? 191.  How  fast  does  it  move  around  its  axis  at  the  equator?  192.  How  is  the 
velocity  of  the  Earth  ascerlained?  193.  Why  are  we  insensible  of  the  Earth's 
motion  ? 


312  HEAT  .AND  COLD. 

CAUSES  OF  THE  HEAT  AND  COLD  OF  THE  SEASONS. 

194.  We  have  seen  that  the  Earth  revolves  round  the  Sun  in 
an  elliptical  orbit,  of  which  the  Sun  is  one  of  the  foci,  and  con- 
sequently that  the  Earth  is  nearer  him,  in  one  part  of  her  orbit 
than  in  another.     From  the  great  difference  we  experience  be- 
tween the  heat  of  Summer  and  that  of  Winter,  we  should  be 
led  to  suppose  that  the  Earth  must  be  much  nearer  the  Sun  in 
the  hot  season  than  in  the  cold.     But  when  we  come  to  inquire 
into  this  subject,  and  to  ascertain  the  distance  of  the  Sun  at  dif- 
ferent seasons  of  the  year,  we  find  that  the  great  source  of  heat 
an'd  light  is  nearest  us  during  the  cold  of  Winter,  and  at  the 
greatest  distance  during  the  heat  of  Summer. 

195.  It  has  been  explained,  under  the  article  Optics,  (39,) 
that  the  angle  of  vision  depends  on  the  distance  at  which  a 
body  of  given  dimensions  is  seen.     Now,  on  measuring  the  an- 
gular dimensions  of  the  Sun,  with  accurate  instruments,  at  dif- 
ferent seasons  of  the  year,  it  has  been  found  that  his  dimensions 
increase  and  diminish,  and  that  these  variations  correspond  ex- 
actly with  the  supposition  that  the  Earth  moves  in  an  elliptical 
orbit. 

196.  If,  for  instance,   his  apparent   diameter   be  taken  in 
March,  and  then  again  in  July,  it  will  be  found  to  have  dimin- 
ished, which  diminution  is  only  to  be  accounted  for,  by  suppos- 
ing that  he  is  at  a  greater  distance  from  the  observer  in  July 
than  in  March.     From  July,  his  angular  diameter  gradually  in- 
creases, till  January,  when  it  again  diminishes,  and  continues  to 
diminish,  until  July.     By  many  observations,  it  is  found,  that 
the  greatest  apparent  diameter  of  the  Sun,  and  therefore  his 
least  distance  from  us,  is  in  January,  and  his  least  diameter,  and 
therefore  his  greatest  distance,  is  in  July. 

197.  The  actual  difference  is  about  three  millions  of  miles, 
the  Sun  being  that  distance  further  from  the  Earth  in  July  than 
in  January.     This,  however,  is  only  about  one-sixtieth  of  his 
mean  distance  from  us;  and  the  difference  we  should  experience 
in   his   heat,   in    consequence    of  this   difference   of  distance, 
will  therefore  be  very  small.     Perhaps  the  effect  of  his  prox- 
imity to  the  Earth  may  diminish,  in  some  small  degree,  the 
severity  of  Winter. 

194.  At  what  season  of  the  year  is  the  Sun  at  the  greatest,  and  at  what  season  the 
least  d'stance  from  the  Earth'?  195.  How  is  it  ascertained  that  the  Earth  moves  in 
an  elliptical  orbit,  by  the  appearance  of  the  Sun  }  196.  When  does  the  Sun  apnpar 
under  the  greatest  apparent  diameter,  and  when  under  the  least!  197.  How  much 
further  is  the  Sun  from  us  in  July  than  in  January  1  What  effect  does  this  difference 
produce  on  the  Earth? 


HEAT    AND    COLD. 


313 


198.  Temperature  of  Summer  and  Winter. — The  heat  of 
Summer,'  and  the  cold  of  Winter,  must  therefore  arise  from  the 
difference  in  the  meridian  altitude  of  the  Sun,  and  in  the  time 
of  his  continuance  above  the  horizon.     In  Summer,  the  sglar 
ra^s  fall  on  the  Earth,  in  nearly  a  perpendicular  direction,  and 
his  powerful  heat  is  then  constantly  accumulated  by  the  long 
days  and  short  nights  of  the  season. 

199.  In  Winter,  on  the  contrary,  the  solar  rays  fall  so  ob- 
liquely on  the  Earth,  as  to  produce  little  warmth,  and  the  small 
effect  they  do  produce  during  the  short  days  of  that  season,  is 
almost  entirely  destroyed  by  the  long  nights  which  succeed. 
The  difference  between  the  effects  of  perpendicular  and  oblique 
rays,  seems  to  depend,  in  a  great  measure,  on  the  different  ex- 
tent of  surface  over  which  they  are  spread. 

200.  When  the  rays  of  the  Sun  are  made  to  pass  through  a 
convex  lens,  the  heat  is  increased,  because  the  number  of  rays 
which  naturally  cover  a  large  surface,  are  then  made  to  cover  a 
smaller  one,  so  that  the  power  of  the  glass  depends  on  the  num- 
ber of  rays  thus  brought  to  a  focus.     If,  on  the  contrary,  the 
rays  of  the  Sun  are  suffered  to  pass  through  a  concave  lens, 
their  natural  heating  power  is  diminished,  because  they  are  dis- 
persed, or  spread  over  a  wider  surface  than  before. 

201.  Summer  and 
Winter  Rays. — Now 
to  apply  these  differ- 
ent effects  to  the  Sum- 
mer and  Winter  rays 
of  the  Sun,  let  us  sup- 
pose that  the  rays  fall- 
ing perpendicularly  on 
a  given  extent  of  sur- 
face,  impart  to  it   a 
certain  degree  of  heat, 
then  it  is  obvious,  that 
if  the  same  number  of 
rays   be   spread  over 
twice    that  extent  of 
surface,  their  heating 
power   would   be  di- 
minished   in    propor- 


FIG.  250. 


Summer  and  Winter  Rays. 


193  How  is  the  heat  of  Summer,  and  the  cold  of  Winter,  accounted  for?  199 
Why  do  the  perpendicular  rays  of  Summer  produce  greater  effects  than  the  oblique 
rays  of  Winter  ?  200.  How  is  this  illustrated  by  the  convex  and  concave  lenses  ' 

14 


814  FIGURE    OP    THE    EARTH. 

tion,  and  that  only  half  the  heat  would  be  imparted.  This  is 
the  effect  produced  by  the  Sun's  rays  in  the  Winter.  They 
fall  so  obliquely  on  the  Earth,  as  to  occupy  nearly  double  the 
spa^ce  that  the  same  number  of  rays  do  in  the  Summer. 

This  is  illustrated  by  Fig.  250,  where  the  number  of  rays, 
both  in  Winter  and  Summer,  are  supposed  to  be  the  same. 
But,  it  will  be  observed,  that  the  Winter  rays,  owing  to  their 
oblique  direction,  are  spread  over  nearly  twice  as  much  surface 
as  those  of  Summer. 

202.  It  may,  however,  be  remarked,  that  the  hottest  season 
is  not  usually  at  the  exact  time  of  the  year,  when  the  Sun  is 
most  vertical,  and  the  days  the  longest,  as  is  the  case  toward  the 
end  of  June,  but  some  time  afterward,  as  in  July  and  August. 

203.  To  account  for  this,  it  must  be  remembered,  that  when 
the  Sun  is  nearly  vertical,  the  Earth  accumulates  more  heat  by 
day  than  it  gives  out  at  night,'  and  that  this  accumulation  con- 
tinues to  increase  after  the  days  begin  to  shorten,  and,  conse- 
quently, the  greatest  elevation  of  temperature  is  some  time  after 
the  longest  days.     For  the  same  reason,  the  thermometer  gen- 
erally indicates  the  greatest  degree  of  heat  at  two  or  three 
o'clock  on  each  day,  and  not  at  twelve  o'clock,  when  the  Sun's 
rays  are  most  powerful. 


FIGURE    OF    THE    EARTH. 


204.  Astronomers  have  proved  that  all  the  planets,  together 
with  their  satellites,  have  the  shape  of  the  sphere,  or  globe,  and 
hence,  by  analogy,  there  was  every  reason  to  suppose  that  the 
Earth  would  be  found  of  the  same  shape ;  and  several  phe- 
nomena tend  to  prove,  beyond  all  doubt,  that  this  is  its  form. 
The  figure  of  the  Earth  is  not,  however,  exactly  that  of  a  globe, 
or  ball,  because  its  diameter  is  about  34  miles  less  from  pole  to 
pole,  than  it  is  at  the  equator.     But  that  its  general  figure  is 
that  of  a  sphere,  or  ball,  is  proved  by  many  circumstances. 

205.  When  one  is  at  sea,  or  standing  on  the  sea-shore,  the 
first  part  of  a  ship  seen  at  a  distance,  is  its  mast.     As  the  vessel 
advances,  the  mast  rises  higher  and  higher  above  the  horizon, 
and  finally  the  hull,  and  whole  ship,  'become  visible.     Now, 
were  the  Earth's  surface  an  exact  plane,  no  such  appearance 


201.  How  is  the  actual  difference  of  the  Summer  and  Winter  rays  shown  ?  202. 
Why  is  not  the  hottest  season  of  the  year  at  the  period  when  the  days  are  longest, 
and  the  Sun  most  vertical  1  203.  How  is  this  accounted  for  ?  204.  What  is  the  gen- 
eral figure  of  the  Earth  1  How  much  less  is  the  diameter  of  the  Earth  at  the  poles 
than  at  the  equator  ?  205.  How  is  the  convexity  of  the  Earth  proved,  by  the  ap 
proach  of  a  ship  at  sea  1  Explain  Fig.  251. 


TIGURE    OF   THE    EARTH.  815 

FIG.  251. 
Tht  Earth's  Convexity 


Spheroidal  Fbrm  of  the  Earth. 

would  take  place,  for  we  should  then  see  the  hull  long  before 
the  mast  or  rigging,  because  it  is  much  the  largest  object. 

It  will  be  plain  by  Fig.  251,  that  were  the  ship,  A,  elevated 
so  that  the  hull  should  be  on  a  horizontal  line  with  the  eye,  the 
whole  ship  would  be  visible,  instead  of  the  topmast,  there  being 
no  reason,  except  the  convexity  of  the  earth,  why  the  whole 
ship  should  not  be  visible  at  A,  as  well  as  at  B. 

206.  We  know,  for  the  same  reason,  that  in  passing  over  a 
hill,  the  tops  of  the  trees  are  seen,  before  we  can  discover  the 
ground  on  which  they  stand ;  and  that  when  a  man  approaches 
from  the  opposite  side  of  a  hill,  his  head  is  seen  before  his  feet. 

It  is  a  well  known  fact  also,  that  navigators  have  set  out  from 
a  particular  port,  and  by  sailing  continually  westward,  have 
passed  around  the  Earth,  and  again  reached  the  port  from  which 
they  sailed.  This  could  never  happen,  were  the  Earth  an  ex- 
tended plain,  since  then  the  longer  the  navigator  sailed  in  one 
direction,  the  further  he  would  be  from  home. 

Another  proof  of  the  spheroidal  form  of  the  Earth,  is  the 
figure  of  its  shadow  on  the  Moon,  during  eclipses,  which  shadow 
is  always  bounded  by  a  circular  line. 

These  circumstances  prove  beyond  all  doubt,  that  the  form  of 
the  Earth  is  globular,  but  that  it  is  not  an  exact  sphere ;  and 
that  it  is  depressed  or  flattened  at  the  poles,  is  shown  by  the 
difference  in  the  lengths  of  pendulums  vibrating  seconds  at  the 
poles,  and  at  the  equator. 

207.  The  compression  of  the  Earth  at  the  poles,  and  the 
consequent  accumulation  of  matter  at  the  equator,  is  considered 
the  effect  of  its  diurnal  revolution,  while  it  was  in  a  soft  or 
plastic  state.     If  a  ball  of  soft  clay,  or  putty,  be  made  to  revolve 

206.  What  other  proofs  of  the  globular  shape  of  the  Earth  are  mentioned  7  207. 
How  is  the  form  of  the  Earth  illustrated  by  experiment  ?  Explain  the  reason  why  a 
plastic  ball  will  swell  at  the  equator,  when  made  to  revolve. 


316 


TIGURE    OF   THE    EARTH. 


rapidly,  by  means  of  a  stick  passing  through  its  center,  as  an 
axis,  it  will  swell  out  in  the  middle,  or  equator,  and  be  de- 
pressed at  the  poles,  assuming  the  precise  figure  of  the  Earth. 

208.  Centrifugal  Force. — The  effects  of  centrifugal  force  are 
very  satisfactorily  illustrated  in  the  following  manner : — 

Two  hoops  of  thin 

iron  are  placed  upon  FIG.  252. 

an  axis  which  passes 
through  their  poles,  as 
shown  by  Fig.  252. 
The  two  poles  of  each 
hoop  cross  each  other 
at  right-angles,  and 
are  fastened  .together, 
and  to  the  axis  at  the 
bottom.  At  the  up- 
per end  they  slide  up 
and  down  on  the  axis, 
which  is  turned  rap- 
idly by  wheel-work  as 
represented.  These 
hoops,  before  the  mo- 
tion begins,  have  an 
oval  form,  but  when 
turned  rapidly,  the 
centrifugal  force  occa- 
sions them  to  expand, 
or  swell  at  the  equator,  while  they  are  depressed  at  the  poles,  the 
two  polar  regions  becoming  no  more  distant  than  A  and  B. 

209.  The  weight  of  a  body  at  the  poles  is  found  to  be  greater 
than  at  the  equator,  not  only  because  the  poles  are  nearer  the 
center  of  the  Earth  than  the  equator,  but  because  the  centrifu- 
gal force  there  tends  to  lessen  its  gravity.     The  wheels  of  ma- 
chines, which  revolve  with  the  greatest  rapidity,  are  made  in 
the  strongest  manner,  otherwise  they  will  fly  in  pieces,  the  cen- 
trifugal force  not  only  overcoming  the  gravity,  but  the  cohesion 
of  their  parts. 

210.  It  has  been  found,  by  calculation,  that  if  the  Earth 
turned  over  once  in  84  minutes  and  43  seconds,  the  centrifugal 


Depressions  of  the  Poles. 


208.  Explain  Fig.  252,  and  show  how  it  illustrates  the  form  of  the  earth.  209.  What 
two  causes  render  the  weights  of  bodies  less  at  the  equator  than  at  the  poles  ?  210. 
What  would  be  the  consequence  on  the  weights  of  bodies  at  the  equator,  did  the 
Earth  turn  over  onee  in  84  minutes  and  43  seconds  ? 


SOLAR   AND    SIDERIAL   TIME.  31 Y 

force  at  the  equator  would  be  equal  to  the  power  of  gravity 
there,  and  that  bodies  would  entirely  lose  their  weight.  If  the 
Earth  revolved  more  rapidly  than  this,  all  the  buildings,  rocks, 
mountains,  and  men,  at  the  equator,  would  not  only  lose  their 
weight,  but  would  fly  away,  and  leave  the  Earth,  as  the  water 
does  from  a  revolving  grindstone. 


SOLAR    AND    SIDERIAL   TIME. 


211.  The  stars  appear  to  go  round  the  Earth  in  23  hours, 
56  minutes,  and  4  seconds,  while  the  Sun  appears  to  perform 
the  same  revolution  in  24  hours,  so  that  the  stars  gain  3  minutes 
ajid  56  seconds  upon  the  Sun  every  day.     In   a   year,  this 
amounts  to  a  day,  or  to  the  time  taken  by  the  Earth  to  per- 
form one  diurnal  revolution.     It  therefore  happens,  that  when 
time  is  measured  by  the  stars,  there  are  366  days  in  the  year, 
or  366  diurnal  revolutions  of  the  Earth ;  while,  if  measured  by 
the  Sun  from   one  meridian  to  another,  there  are  only  365 
whole  days  in  the  year.     The  former  are  called  the  siderial, 
and  the  latter  solar  days. 

212.  If  the  Earth  had  only  a  diurnal  motion,  her  revolution, 
in  respect  to  the  Sun,  would  coincide  exactly  with  the.  same 
revolution  in  respect  to  the  stars ;  but  while  she  is  making  one 
revolution  on  her  axis  toward  the  east,  she  advances  in  the 
same  direction  about  one  degree  in  her  orbit,  so  that  to  bring 
the  same  meridian  toward  the  Sun,  she  must  make  a  little  more 
than  one  entire  revolution. 

213.  Thus,  the  Earth  must  complete  one  revolution,  and  a 
portion  of  a  second  revolution,  equal  to  the  space  she  has  ad- 
vanced in  her  orbit,  in  order  to  bring  the  same  meridian  back 
again  to  the  Sun.     This  small  portion  of  a  second  revolution 
amounts  daily  to  the  365th  part  of  her  circumference,  and 
therefore,  at  the  end  of  the  year,  to  one  entire  rotation,  and 
hence,  in  365  days,  the  Earth  actually  turns  on  her  axis  366 
times.     Thus,   as  one  complete  rotation  forms  a  siderial  day, 
there  must,  in  the  year,  be  one  siderial,  more  than  there  are 
solar  days,  one  rotation  of  the  Earth,  with  respect  to  the  Sun, 
being  lost,  by  the  Earth's  yearly  revolution.     The  same  loss  of 


211.  The  stars  appear  to  move  round  the  Earth  in  less  time  than  the  Sun  ;  what 
does  the  difference  amount  to  in  a  year  1  What  is  the  year  measured  by  a  star 
called?  What  is  that  measured  by  the  Sun  called?  212."  Had  the  Earth 'only  a 
diurnal  revolution,  would  the  siderial  and  solar  time  agree?  213.  How  many  times 
does  the  Earth  turn  on  her  axis  in  a  vear  7  Why  does  sh«  turn  more  times  than 
there  are  days  in  the  year? 


318  TIME. 

a  day  happens  to  a  traveler,  who,  in  passing  round  the  Earth 
to  the  west,  reckons  his  time  by  the  rising  and  setting  of  the 
Sun.  If  he  passes  round  toward  the  east,  he  will  gain  a  day 
for  the  same  reason. 

EQUATION   OF   TIME. 

214. 'As  the  motion  of  the  Earth  about  its  axis  is  perfectly 
uniform,  the  siderial  days  are  exactly  of  the  same  length,  in  all 
parts  of  the  year.  But  as  the  orbit  of  the  Earth,  or  the  appa- 
rent path  of  the  Sun,  is  inclined  to  the  Earth's  axis,  and  as  the 
Earth  moves  with  different  velocities  in  different  parts  of  its 
orbit,  the  solar,  or  natural  days,  are  sometimes  greater  and 
sometimes  less  than  24  hours,  as  shown  by  an  accurate  clock. 
The  consequence  is,  that  a  true  sun-dial,  or  noon  mark,  and  a 
true  time-piece,  agree  with  each  other  only  a  few  times  in  a 
year.  The  difference  between  the  sun-dial  and  clock,  thus 
shown,  is  called  the  equation  of  time. 

215.  The  difference  between  the  Sun  and  a  well  regulated 
clock,  thus  arises  from  two  causes,  the  inclination  of  the  Earth's 
axis  to  the  ecliptic,  and  the  elliptical  form  of  the  Earth's  orbit. 

That  the  Earth  moves  in  an  ellipse,  and  that  its  motion  is 
more  rapid  sometimes  than  at  others,  as  well  as  that  the  Earth's 
axis  is  inclined  to  the  ecliptic,  have  already  been  explained  and 
illustrated.  It  remains,  therefore,  to  show  how  these  two  com- 
bined causes,  the  elliptical  form  of  the  orbit,  and  the  inclination 
of  the  axis,  produce  the  disagreement  between  the  Sun  and 
clock. 

MEAN   TIME. 

216.  Equal,  or  mean  time,  is  that  which  is  reckoned  by  a 
clock,  supposed  to  indicate  exactly  24  hours,  from  12  o'clock  on 
one  day,  to  12  o'clock  on  the  next  day.     Apparent  time,  is 
that  which  is  measured  by  the  apparent  motion  of  the  Sun  in 
the  heavens,  as  indicated  by  a  meridian  line,  or  sun-dial. 

217.  Were  the  Earth's  orbit  a  perfect  circle,  and  her  axis 
perpendicular  to  the  plane  of  this  orbit,  the  days  would  be  of  a 
uniform  length,  and  there  would  be  no  difference  between  the 
clock  and  the  Sun  ;  both  would  indicate  12  o'clock  at  the  same 


214.  Why  are  the  solar  days  sometimes  greater,  and  sometimes  less,  than  24 
hours  1  What  is  the  difference  between  the  time  of  a  sun-dial  and  clock  called  ?  215. 
What  are  the  causes  of  the  difference  between  the  Sun  and  clock.  216.  What  is 
meant  by  equal,  or  mean  time?  217.  Were  the  Earth's  orbit  a  perfect  circle,  and 
aer  axis  perpendicular  to  its  plane,  what  would  be  the  effect  on  tims  1 


MOON.  319 

time,  on  every  day  in  the  year.  But  on  account  of  the  inclina- 
tion of  the  Earth's  axis  to  the  ecliptic,  unequal  portions  of  the 
Sun's  apparent  path  through  the  heavens  will  pass  any  meridian 
in  equal  times. 

218.  Thus  the  elliptical  form  of  the  Earth's  orbit,  her  unequal 
motions   and   the   inclination  of  her  axis,  would  prevent  the 
agreement  of  the  Sun  and  clock,  except  when  the  Earth  is  at 
the  greatest  distance  from  the  Sun,  which  is  on  the  1st  of  July, 
and  when  she  is  at  the  least  distance,  which  is  on  the  1st  of 
January.     From  these  causes  the  Sun  would  be  faster  than  the 
clock,  from  the  1st  of  July  to  the  1st  of  January,  and  then 
slower  than  the  clock,  from  the   1st  of  January  to  the  1st  of 
July. 

Now  these  two  causes,  which  result  from  sources  which  can 
not  be  here  explained,  counteract  each  other,  so  that  the  Sun 
and  clock  agree  only  when  they  coincide,  or  balance  each  other, 
which  takes  place,  on,  or  about  the  loth  of  April,  the  15th  of 
June,  the  31st  of  August  and  the  24th  of  December. 

On  these  days  the  Sun  and  clock,  keeping  exact  time,  coin- 
cide, or  as  the  Almanac  says,  are  even. 

219.  The  greatest  differences  between  the  Sun  and  clock,  are 
on  the  1st  of  November,  when  the  clock  is  16£  minutes  too 
fast,  and  on  the  10th  of  February,  when  it  is  14  minutes  too 
slow. 

THE    MOON. 

220.  While  the  Earth  revolves  round  the  Sun,  the  Moon 
revolves  round  the  Earth,  completing  her  revolution  once  in  2*7 
days,  7  hours  and  43  minutes,  and  at  the  distance  of  240,000 
miles  from  the  Earth.     The  period  of  the  Moon's  change,  that 
is,  from  new  Moon  to  new  Moon  againy  is  29  days,  12  hoursy 
and  44  minutes. 

221.  The  time  of  the  Moon's  revolution  round  the  Earth  is 
called  her  periodical  month ;  and  the    time  from    change  to 
change  is  called  her  synodical  month.     If  the  Earth  had  no  an- 
nual motion,  these  two  periods  would  be  equal,  but  because  the 
Earth  goes  forward  in  her  orbit,  while  the  Moon  goes  round 
the  Earth,  the  Moon  must  go  as  much  further,  from  change  to 
change,  to  make  these  periods  equal,  as  the  Earth  goes  forward 

218.  What  prevents  the  agreement  of  the  Sun  and  clock?  When  do  the  Snn  and 
clock  agree  ?  219.  Wtien  do  they  differ  most  ]  220.  What  is  the  period  of  the  Moon's 
revolution  round  the  Earth  ?  What  is  the  period  from  new  Moon  to  new  Moon 
again  ?  221.  What  are  these  two  periods  called  7  Why  are  uot  the  periodical  and 
ty nodical  months  equal  1 


320  MOON. 

during  that  time,  which  is  more  than  the  twelfth  part  of  her 
orbit,  there  being  more  than  twelve  lunar  periods  in  the  year. 

222.  Illustration  by  the  Hands  of  a    Watch. — These    two 
revolutions  maybe  familiarly  illustrated  by  the  motions  of  the 
hour  and  minute  hands  of  a  watch.     Let  us  suppose  the  12 
hours  marked  on  the  dial  plate  of  a  watch  to  represent  the  12 
signs  of  the  zodiac  through  which  the  Sun  seems  to  pass  in  his 
yearly  revolution,  while  the  hour  hand  of  the  watch  represents 
the  Sun,  and  the  minute  hand  the  Moon.     Then,  as  the  hour 
hand  goes  around  the  dial  plate  once  in  12  hours,  so  the  Sun 
apparently  goes  around  the  zodiac  once  in  twelve  months ;  and 
as  the  minute  hand  makes  12  revolutions  to  one  of  the  hour 
hand,  so  the  Moon  makes  12  revolutions  to  one  of  the  Sun. 
But  the  Moon,  or  minute  hand,  must  go  more  than  once  round, 
from  any  point  on  the  circle,  where  it  last  came  in  conjunction 
with  the  Sun,  or  hour  hand,  to  overtake  it  again,  since  the  hour 
hand  will  have  moved  forward  of  the  place  where  it  was  last 
overtaken,  and  consequently  the  next  conjunction  must  be  for- 
ward of  the  place  where  the  last  happened.     During  an  hour, 
the  hour  hand  describes  the  twelfth  part  of  the  circle,  but  the 
minute  hand  has  not  only  to  go  round  the  whole  circle  in  an 
hour,  but  also  such  a  portion  of  it  as  the  hour  hand  has  moved 
forward  since  they  last  met.     Thus,  at  12  o'clock,  the  hands  are 
in  conjunction;  the  next  conjunction  is  5  minutes  27  seconds 
past  I  o'clock;  the  next,  10  min.  54  sec.  past  II  o'clock;  the 
third,  16  min.  21  sec.  past  III;  the  4th,  21  min.  49  sec.  past 
IV;  the  5th,  27  min.  10  sec.  past  V;  the  6th,  32  min.  43  sec. 
past  VI;-  the  7th,  38  min.   10  sec.  past  VII ;  the  8th,  43  min. 
38  sec.  past  VIII;  the  9th,  49  min.  5  sec.  past  IX;  the  10th, 
54  min.  32  sec.  past  X ;  and  the  next  conjunction  is  at  XII. 

223.  The  same  principle  is  true  in  respect  to  the  Moon ;  for 
as  the  Earth  advances  in.  its  orbit,  it  takes  the  Moon  2  days,  5 
hours  and   1  minute  longer  to  come  again  in  conjunction  with 
the  Sun,  than  it  does  to  make  her  monthly  revolution  round 
the  Earth  ;  and  this  2  days  5  hours  and  1  minute  being  added 
to  27  days  7  hours  and  43  minutes,  the  time  of  the  periodical 
revolution,  makes  29  days  12  hours  and  44  minutes,  the  period 
of  her  synodical  revolution. 

224.  We  only  see  one  Side  of  the  Moon. — The  Moon  always 

222.  How  are  these  two  revolutions  of  the  Moon  illustrated  by  the  two  hands  of  a 
watch?  Mention  the  time  of  several  conjunctions  between  the  two  hands  of  a 
watch.  223.  How  much  longer  does  it  take  the  Moon  to  comg  again  in  conjunction 
with  the  Sun,  than  it  does  to  perform  her  periodical  revolution?  224.  Plow  is  it 
proved  that  the  Moon  makes  but  one  revolution  on  her  axis,  as  she  passes  around  the 
Earth  7 


MOOIC.  321 

presents  the  same  side,  or  face,  toward  the  Earth,  and  hence  it 
is  evident  that  she  turns  on  her  axis  but  once,  while  she  is  per- 
forming one  revolution  round  the  Earth,  so  that  the  inhabitants 
of  the  Moon  have  but  one  day  and  night  in  the  course  of  a 
lunar  month. 

225.  One  half  of  the  Moon  is  never  in  the  dark,  because 
when  this  half  is  not  enlightened  by  the  Sun,  a  strong  light  is 
reflected   to  her  from  the  Earth,   during  file  Sun's   absence. 
The  other  half  of  the  Moon  enjoys  alternately  two  weeks  of  the 
Sun's  light,  and  two  weeks  of  total  darkness. 

PHASES  OF  THE  MOON. — One  of  the  most  interesting  circum- 
stances to  us,  respecting  the  Moon,  is  the  constant  changes 
which  she  undergoes,  in  her  passage  around  the  Earth.  When 
she  first  appears,  a  day  or  two  after  her  change,  we  can  see 
only  a  small  portion  of  her  enlightened  side,  which  is  in  the 
form  of  a  crescent ;  and  at  this  time  she  is  commonly  called 
new  Moon.  From  this  period  she  goes  on  increasing,  or  show- 
ing more  and  more  of  her  face,  every  evening,  until  at  last  she 
becomes  round,  and  her  face  is  fully  illuminated.  She  then 
begins  again  to  decrease,  by  apparently  losing  a  small  section 
of  her  face,  and  the  next  evening  another  small  section  from 
the  same  part,  and  so  on,  decreasing  a  little  every  day,  until  she 
entirely  disappears ;  and  having  been  absent  a  day  or  two,  re- 
appears in  the  form  of  a  crescent,  or_new  Moon,  as* before. 

226.  When  the  Moon  disappears7  she  is  said  to  be  in  con- 
junction, that  is,  she  is  in  the  same  direction  from  us  with  the 
Sun.     When  she  is  full,  she  is  said  to  be  in  opposition,  that  is, 
she  is  in  that  part  of  the  heavens  opposite  to  the  Sun,  as  seen 
by  us. 

227.  The  different  appearances  of  the  Moon  from  new  iofull, 
and  from  full  to  change,  are  owing  to  her  presenting  different 
portions  of  her  enlightened  surface  toward  us  at  different  times. 
These  appearances  are  called  phases  of  the  Moon,  and  are  easily 
accounted  for,  and  understood  by  the  following  figure. 

228.  Let  S,  Fig.  253,  be  the  Sun,  E  the  Earth,  and  A,  B,  C, 
D,  F,  the  Moon  in  different  parts  of  her  orbit.     Now  when  the 
Moon  changes,  or  is  in  conjunction  with  the  Sun,  as  at  A,  her  dark 
side  is  turned  toward  the  Earth,  and  she  is  invisible,  as  repre- 

225.  One  half  of  the  Moon  is  never  in  the  dark  ;  explain  why  this  is  so.  How  long 
is  the  day  and  night  at  thf  other  half?  How  is  it  shown  that  the  Moon  shines  only 
by  reflected  light  ?  2C6.  When  is  the  Moon  said  to  be  in  conjunction  with  the  Sun, 
and  when  in  opposition  to  the  Sun  1  227.  What  are  the  phases  of  the  Moon  1  228 
D-scr  b>v  Fijf.  *>3,  and  show  how  the  Moon  passes  from  change  to  full,  and  from 
full  to  change.  -l* 


'322 


Phases  of  the  Moon. 

sented  at  a.  The  Sun  always  shines  on  one  half  of  the  Moon, 
in  every  direction,  as  represented  at  A  and  B,  on  the  inner 
circle ;  but  we  at  the  Earth  can  see  only  such  portions  of  the 
enlightened  part  as  are  turned  toward  us.  After  her  change, 
when  she  has  moved  from  A  to  B,  a  small  part  of  her  illumi- 
nated side  comes  in  sight,  and  she  appears  horned,  as  at  6,  and 
is  then  called  the  new  Moon.  When  she  arrives  at  C,  several 
days  afterward,  one  half  of  her  disc  is  visible,  and  she  appears 
as  at  c,  her  appearance  being  the  same  in  both  circles.  At  this 
point  she  is  said  to  be  in  her  first  quarter,  because  she  has 
passed  through  a  quarter  of  her  orbit,  and  is  90  degrees  from 
the  place  of  her  conjunction  with  the  Sun.  At  D,  she  shows  us 
still  more  of  her  enlightened  side,  and  is  then  said  to  appear 
gibbous,  as  at  d.  When  she  comes  to  F,  her  whole  enlightened 
side  is  turned  toward  the  Earth,  and  she  appears  in  all  the 
splendor  of  the  full  Moon.  During  the  other  half  of  her  revo- 
lution she  daily  shows  less  and  less  of  her  illuminated  side, 
until  she  again  becomes  invisible  by  her  conjunction  with  the 
Sun.  Thus  in  passing  from  her  conjunction  a,  to  her  full  <?,  the 
Moon  appears  every  day  to  increase,  while  in  going  from  her 
full  to  her  conjunction  again,  she  appears  to  us  constantly  to 
decrease,  but  as  seen  from  the  Sun,  she  appears  always  full. 

229.  How  the  Earth  appears  at  the  Moon. — The  earth,  seen 
by  the  inhabitants  of  the  Moon,  exhibits  the  same  phases  that 

229.  What  is  Paid  concerning  the  phases  of  the  Earth,  as  seen  from  the  Moon  1 
When  does  the  Earth  appear  full  at  the  Moon  1 


ECLIPSES.  328 

the  Moon  does  to  us,  but  in  a  contrary  order.  When  the  Moon 
is  in  her  conjunction,  and  consequently  invisible  to  us,  the 
Earth  appears  full  to  the  people  of  the  Moon,  and  when  the 
Moon  is  full  to  us,  the  Earth  is  dark  to  them. 

230.  The  Earth  shines  upon  the  Moon. — That  the  Earth 
shines  upon  the  Moon,  as  the  Moon  does  upon  us,  is  proved  by 
the  fact  that  the  outline  of  her  disc  may  be^seen,  when  only  a 
part  of  it  is  enlightened  by  the  Sun.     Thus  when  the  sky  is 
clear,  and  the  Moon  only  two  or  three  days  old,  it  is  not  un- 
common to  see  the  brilliant  new  Moon,  with  her  horns  enlight- 
ened by  the  Sun,  and  at  the  same  time  the  old  Moon  faintly 
illuminated  by  reflection  from  the  Earth.     This  phenomenon  is 
sometimes  called  "  the  old  Moon  in  the  new  Moon's  arms." 

231.  It  was  a  disputed  point  among  former  astronomers, 
whether  the  Moon  has  an   atmosphere ;  but  the  more  recent 
discoveries  have  decided  that  she  has  an  atmosphere,  though 
there  is  reason  to  believe  that  it  is  much  less  dense  than  ours. 

232.  Surface  of  the  Moon. — When  the  Moon's  surface  is 
examined  through   a  telescope,  it  is  found  to  be  wonderfully 
diversified,  for  besides  the  dark  spots  perceptible  to  the  naked 
eye,  there  are  seen  extensive  valleys,  and  long  ridges  of  highly 
elevated  mountains. 

Some  of  these  mountains,  according  to  Dr.  Herschel,  are  4 
miles  high,  while  hollows  more  than  three  miles  deep,  and 
almost  exactly  circular,  appear  excavated  on  the  plains.  As- 
tronomers have  been  at  vast  labor  to  enumerate,  figure,  and  de- 
scribe the  mountains  and  spots  on  the  surface  of  the  Moon,  so 
that  the  latitude  and  longitude  of  about  100  spots  have  been 
ascertained,  and  their  names,  shapes,  and  relative  positions  given. 
A  still  greater  number  of  mountains  have  been  named,  and 
their  heights  and  the  lengths  of  their  bases  detailed. 

233.  The  deep  caverns,  and  broken  appearance  of  the  Moon's 
surface,   long  since  induced  astronomers  to  believe  that   such 
effects  were  produced  by  volcanoes,  and  more  recent  discoveries 
have  seemed  to  prove  that  this  suggestion  was  not  without 
foundation. 


234.  Every  planet  and  satellite  in  the   Solar  System,  is  il- 
luminated by  the   Sun,  and  hence  they  cast  shadows  in   the  di- 

230.  How  is  it  known  that  the  Earth  shines  upon  the  Moon,  as  the  Moon  does  upon 
us!  231.  What  is  said  concerning  the  Moon's  atmosphere'?  232.  How  high  are 
some  of  the  mountains,  and  how  deep  the  caverns  of  the  Moon  1  233.  What  is  said 
concerning  the  volcanoes  of  the  Moon  7  234.  What  is  a  lunar,  and  what  a  solar 
eclipse  1 


324  LUNAR   ECLIPSES. 

rection  opposite  to  him,  just  as  the  shadow  of  a  man  reaches 
from  the  Sun. 

235.  Eclipses,  what. — Eclipses  are  of  two  kinds,  namely 
Lunar,  an  eclipse  of  the  Moon,  and  Solar,  an  eclipse  of  the 
Sun.  The  first  is  occasioned  by  the  shadow  of  the  Earth  on 
the  Moon,  and  the  second  by  the  shadow  of  the  Moon  on  the 
Earth. 

Hence,  in  both  cases,  the  two  planets  and  the  Sun  must  be  in 
nearly  a  straight  line  with  respect  to  each  other.  In  eclipses 
of  the  Moon,  the  Earth  is  between  the  Sun  and  Moon ;  and  in 
eclipses  of  the  Sun,  the  Moon  is  between  the  Earth  and  Sun. 


LUNAR    ECLIPSES. 


236.  When  the  Moon  falls  into  the  shadow  of  the  Earth, 
the  rays  of  the  Sun  are  intercepted,  or  hid  from  her,  and  she 
then  becomes  eclipsed. 

When  the  Earth's  shadow  covers  only  a  part  of  her  face,  as 
seen  by  us,  she  suffers  only  a  partial  eclipse,  one  part  of  her 
disc  being  obscured,  while  the  other  part  reflects  the  Sun's  light. 
But  when  her  whole  surface  is  obscured  by  the  Earth's  shadow, 
she  then  suffers  a  total  eclipse,  and  of  a  duration  proportionate 
to  the  distance  she  passes  through  the  Earth's  shadow. 

FIG.  254. 


Eclipse  of  the  Moon. 

237.  Fig.  254  represents  a  total  lunar  eclipse;  the  Moon 
being  in  the  midst  of  the  Earth's  shadow.  Now  it  will  be  ap- 
parent that  in  the  situation  of  the  Sun,  Earth,  and  Moon,  as 
represented  in  the  figure,  this  eclipse  will  be  visible  from  all 
parts  of  that  hemisphere  of  the  Earth  which  is  next  the  Moon, 
and  that  the  Moon's  disc  will  be  equally  obscured,  from  what- 
ever point  it  is  seen.  When  the  moon  passes  through  only  a 

235.  What  occasions  the  lunar,  and  what  the  solar  eclipse?  236.  What  is  meanf 
DY  a  partial,  and  what  by  a  total  eclipse  ?  237.  Why  is  the  same  eclipse  total  at  one 
place,  and  only  partial  at  another  1 


SOLAR   ECLIPSES.  325 

part  of  the  Earth's  shadow,  then  she  suffers  only  a  partial 
eclipse,  but  this  is  also  visible  from  the  whole  hemisphere  next 
the  Moon.  It  will  be  remembered  that  lunar  eclipses  happen- 
only  at  full  Moon,  the  Sun  and  Moon  being  in  opposition,  and 
the  Earth  between  them. 


SOLAR    ECLIPSES. 


238.  When  the  Moon  passes  between  the  Earth  and  Sun, 
there  happens  an  eclipse  of  the  Sun,  because  then  the  Moon's 
shadow  falls  upon  the  Earth. 

239.  A  total  eclipse  of  the  Sun  happens  often,  but  when  it 
occurs,  the  total  obscurity  is  confined  to  a  small  part  of  the 
Earth ;  since  the  dark  portion  of  the  Moon's  shadow  never  ex- 
ceeds 200  miles  in  diameter  on  the  Earth.     But  the  Moon's 
partial  shadow,  or  penumbra,  may  cover  a  space  on  the  Earth 
of  more  than   4,000  miles  in  diameter,  within  all  which  space 
the  Sun  will   be  more  or  less  eclipsed.     When  the  penumbra 
first  touches  the  Earth,  the  eclipse  begins  at  that  place,  and 
ends  when  the  penumbra  leaves  it.     But  the  eclipse  will  be 
total  only  where  the  dark  shadow  of  the  Moon  touches  the 
Earth. 


FIG.  255. 


Eclipse  of  the  Sun. 

Fig.  255,  represents  an  eclipse  of  the  Sun,  without  regard  to 
the  penumbra,  that  it  may  be  observed  how  small  a  part  of  the 
Earth  the  dark  shadow  of  the  Mooji  covers.  To  those  who 
live  within  the  limits  of  this  shadow,  the  eclipse  will  be  total, 
while  to  those  who  live  in  any  direction  around  it,  and  within 
reach  of  the  penumbra,  it  will  be  only  partial. 

240.  Solar  eclipses  are  called  annular,  from  annulus,  a  ring, 

238.  Why  is  a  total  eclipse  of  the  Snn  confined  to  so  small  a  part  of  the  Earth  * 
239.  What  is  meant  by  penumbra?  What  will  be  the  difference  in  the  aspect  of  the 
eclipse,  whether  the  observer  stands  within  the  dark  shadow,  or  only  within  the  pe- 
numbra? 240.  What  is  meant  by  annular  eclipses  ]  Are  annular  eclipses  ever  total 
in  any  part  of  the  Earth  ?  In  annular  eclipses,  what  part  of  the  Moon's  shadow 
reaches  the  Earth  ? 


326  SOLAR   ECLIPSES. 

when  the  Moon  passes  across  the  center  of  the  Sun,  hiding  all 
his  light,  with  the  exception  of  a  ring  on  his  outer  edge,  which 
the  Moon  is  too  small  to  cover  from  the  position  in  which  it  is 
seen. 

241.  Umbra  and  Penumbra. — A  solar  eclipse,  with  the  pe- 
numbra, or  light  and  shadow,  D  C,  and  the  umbra,  or  dark 
shadow,  O,  is  seen  in  Fig.  256. 

When  the  Moon  is  at  its  greatest  distance  from  the  Earth,  its 
shadow  M  0,  sometimes  terminates  before  it  reaches  the  Earth, 
and  then  an  observer  standing  directly  under  the  point  O,  will 
see  the  outer  edge  of  the  Sun,  forming  a  bright  ring  around  the 
circumference  of  the  Moon,  thus  forming  an  annular  eclipse. 

FIG.  256. 


Umbra  and  Penumbra. 

The  penumbra  D  C,  is  only  a  partial  interception  of  the  Sun'a 
rays,  and  in  annular  eclipses  it  is  this  partial^hadow  only  which 
reaches  the  Earth,  while  the  umbra,  or  dark  shadow,  terminates 
in  the  air.  Hence  annular  eclipses  are  never  total  in  any  part 
of  the  Earth.  The  penumbra,  as  already  stated,  may  cover 
more  than  4,000  miles  of  space,  while  the  umbra  never  covers 
more  than  200  miles  in  diameter ;  hence  partial  eclipses  of  the 
Sun  may  be  seen  by  a  vast  number  of  inhabitants,  while  com- 
paratively few  will  witness  the  total  eclipse. 

242.  When  there  happens  a  total  solar  eclipse  to  us,  we  are 
eclipsed  to  the  Moon,  and  wJien  the  Moon  is  eclipsed  to  us,  an 
eclipse  of  the  Sun  happens  to  the  Moon.  To  the  Moon,  an 
eclipse  of  the  Earth  can  never  be  total,  since  her  shadow  cover? 
only  a  small  portion  of  the  Earth's  surface.  Such  an  eclipse, 
therefore,  at  the  Moon,  appears  only  as  a  dark  spot  on  the  face 
of  the  Earth  ;  but  when  the  Moon  is  eclipsed  to  us,  the  Sun  is 
partially  eclipsed  to  the  Moon  for  several  hours  longer  than  the 
Moon  is  eclipsed  to  us. 


241 .  What  do  penumbra,  and  umbra,  mean  1  242.  What  is  said  concerning  eclipses 
of  the  Earth,  as  seen  from  the  Moon  7 


TIDES.  327 


THE   TIDES. 

243.  The  ebbing  and  flowing  of  the  sea,  which  regularly 
takes  place  twice  in  24  hours,  are  called  the  tides. 

244.  The  cause  of  the  tides,  is  the  attraction  of  the  Sun  and 
Moon,  but  chiefly  of  the  Moon,  on  the  waters  of  the  ocean.     In 
virtue  of  the  universal  principle  of  gravitation,  heretofore  ex- 
plained, the  Moon,  by  her  attraction,  draws,  or  raises  the  water 
f     ~rd  her,  but  because  the  power  of  attraction  diminishes  as 
uie  squares  of  the  distances  increase,  the  waters,  on  the  oppo- 
site side  of  the  Earth,  are  not  so  much  attracted  as  they  are  on 
the  side  nearest  the  Moon. 

245.  The  want  of  attraction,  together  with  the  gfeater  cen- 
trifugal force  of  the  Earth  on  its  opposite  side,  produced  in  con- 
sequence of  its  greater  distance  from  the  common  center  of 
gravity,  between  the  Earth  and  Moon,  causes  the  waters  to  rise 
on  the  opposite  side,  at  the  same  time  that  they  are  raised  by 
direct  attraction  on  the  side  nearest  the  Moon. 

Thus  the  waters  are  constancy  elevated  on  the  sides  of  the 
Earth  opposite  to  each  other  above  their  common  level,  and 
consequently  depressed  at  opposite  points  equally  distant  from 
these  elevations. 

FIG.  257. 


Illustration  of  the  Tides. 

246.  LetM,  Fig.  257,  be  the  Moon,  and  E  the  Earth,  covered 
with  water.     As  the  Moon  passes  round  the  Earth,  its  solid 
and  fluid  parts  are  equally  attracted  by  her  influence  according 
to  their  densities ;  but  while  the  solid  parts  are  at  liberty  to 
move  only  as  a  whole,  the  water  obeys  the  slightest  impulse, 
and  thus  tends  toward  the  Moon  where  her  attraction  is  the 
strongest.     Consequently,  the  waters  are  perpetually  elevated 
immediately  under  the  Moon. 

247.  If,  therefore,  the  Earth  stood  still,  the  influence  of  the 

243.  What  are  the  tides  ?  244.  What  is  the  cause  of  the  tides?  245.  What  causes 
the  tide  to  ri.se  on  the  side  of  the  Earth  opposite  to  the  Moon  ?  246.  Explain  Fig.  257. 
247.  If  the  Earth  stood  still,  the  tides  would  rise  only  as  the  Moon  passes  round  the 
Earth  ;  what  then  causes  the  tides  to  rise  twice  in  24  hours  ? 


328  TIDES. 


Moon's  attraction  would  raise  the  tides  only  as  she  passed  round 
the  Earth.  But  as  the  Earth  turns  on  her  axis  every  24  hours, 
and  as  the  waters  nearest  the  Moon,  as  at  A,  are  constantly 
elevated,  they  will,  in  the  course  of  24  hours,  move  round  the 
whole  Earth,  and  consequently  from  this  cause  there  will  be 
high  water  at  every  place  once  in  24  hours.  As  the  elevation 
of  the  waters  under  the  Moon  causes  their  depression  at  90  de- 
gress distance  on  the  opposite  sides  of  the  Earth,  D  and  C,  the 
point  C  will  come  to  the  same  place,  by  the  Earth's  diurnal 
revolution,  six  hours  after  the  point  A,  because  C  is  one  quarter 
of  the  circumference  of  the  Earth  from  the  point  A,  and  there- 
fore, there  will  be  low  water  at  any  given  place  six  hours  after 
it  was  high  "water  at  that  place. 

248.  But  while  it  is  high  water  under  the  Moon,  in  conse- 
quence of  her  direct  attraction,  it  is  also  high  water  on  the  op- 
posite  side  of  the  Earth  in  consequence   of  her   diminished 
attraction,  and  the  Earth's  centrifugal  motion,  and  therefore  it 
will  be  high  water  from  this  cause  twelve  hours  after  it  was  high 
water  from  the  former  cause,  and  six  hours  after  it  was  low 
water  from  both  causes. 

249.  But   while  the  Earth  turns   on  her  axis,  the   Moon 
advances  in  her  orbit,  and  consequently  any  given  point  on  the 
Earth  will  not  come  under  the  Moon  on  one  day  so  soon  as  it 
did  on  the  day 'before.     For  this  reason,  high  or  low  water  at 
any  place  comes  about  fifty  minutes  later  on  one  day  than  it 
did  the  day  before. 

Thus  far  we  have  considered  no  other  attractive  influence  ex- 
cept that  of  the  Moon,  as  affecting  the  waters  of  the  ocean. 
But  the  Sun,  as  already  observed,  has  an  effect  upon  the  tides, 
though  on  account  of  his  great  distance,  his  influence  is  small 
when  compared  with  that  of  the  Moon. 

250.  When  the  Sun  and  Moon  are  in  conjunction,  as  repre- 
sented in  Fig.  257,  which  takes  place  at  her  change,  or  when 
they  are  in  opposition,  which  takes  place  at  full  Moon,  then 
their  forces  are  united,  or  act  on  the  waters  in  the  same  direc- 
tion, and  consequently  the  tides  are  elevated  higher  than  usual, 
and  on  this  account  are  called  spring  tides. 

251.  Neap  Tides. — But  when  the  Moon  is  in  her  quadra- 
tures, or  quarters,  the  attraction  of  the  Sun  tends  to  counteract 


248.  When  it  is  high  water  under  the  Moon  by  her  attraction,  what  is  the  cause  of 
high  water  on  the  opposite  side  of  the  Earth,  at  the  same  time  1  249  Why  are  the 
tides  about  fifty  minutes  later  every  day?  250.  What  produces  spriiig  tides? 
Where  must  the  Moon  be  in  respect  to  the  Bun,  to  produce  spring  tides?  251 
What  is  the  occasion  of  neap  tides  ? 


LATITUDE    AND    LONGITUDE.  329 

that  of  the  Moon,  and  although  his  attraction  does  not  elevate 
the  waters  and  produce  tides,  his  influence  diminishes  that  of 
the  Moon,  and  consequently  the  elevation  of  the  waters  are  less 
when  the  Sun  and  Moon  are  so  situated  in  respect  to  each  other, 
than  when  they  are  in  conjunction  or  opposition. 

This  effect  is  represented  by  Fig.  258,  where  the  elevation 
of  the  tides  at  C  and  D  is  produced  by  the  causes  already  ex- 
plained ;  but  their  elevation  is  not  so  great  as  in  Fig.  257,  since 
the  influence  of  the  Sun  acting  in  the  direction  A  B,  tends  to 
counteract  the  Moon's  attractive  influence.  These  small  tides 
are  called  neap  tides,  and  happen  only  when  the  Moon  is  in  her 
quadratures. 

FIG.  258. 


Small,  or  Neap  Tides. 

The  tides  are  not  at  their  greatest  heights  at  the  time  when 
the  Moon  is  at  its  meridian,  but  some  time  afterward,  because 
the  water,  having  a  motion  forward,  continues  to  advance  by  its 
own  inertia,  some  time  after  the  direct  influence  of  the  Moon 
has  ceased  to  affect  it. 

LATITUDE    AND   LONGITUDE. 

252.  Latitude  is  the  distance  from  the  equator  in  a  direct 
line,  north  or  south,  measured  in  degrees  and  minutes. 

The  number  of  degrees  is  90  north,  and  as  many  south,  each 
line  on  which  these  degrees  are  reckoned  running  from  the 
equator  to  the  poles.  Places  at  the  north  of  the  equator  are  in 
north  latitude,  and  those  south  of  the  equator  are  in  south  lat- 
itude. The  parallels  of  latitude  are  imaginary  lines  drawn 
parallel  to  the  equator,  either  north  or  south,  and  hence  every 
place  situated  on  the  same  parallel,  is  in  the  same  latitude  be- 

252.  What  is  latitude  ?  How  many  degrees  of  latitude  are  there  ?  How  far  do  the 
lines  of  latitude  extend  ?  What  is  meant  by  north  and  south  latitude  ?  What  are 
the  parallels  of  latitude  7 


330 


LATITUDE    AND    LONGITUDE. 


cause  every  such  place  must  be  at  the  same  distance  from  the 
equator.  The  length  of  a  degree  of  latitude  is  60  geographical 
mile*. 

253.  Longitude  is  the  distance  measured  in  degrees  and  min- 
utes, either  east  or  west,  from  any  given  point  on  the  equator,  or 
on  any  parallel  of  latitude.     Hence  the  lines,  or  meridians  of 
longitude,  cross  those  of  latitude  at  right-angles.     The  degrees 
of  longitude  are  180  in  number,  its  lines  extending  half  a  circle 
to  the  east,  and  half  a  circle  to  the  west,  from  any  given  me- 
ridian, so  as  to  include  the  whole  circumference  of  the  Earth. 
A  degree  of  longitude,  at  the  equator,  is  of  the  same  length  as 
a  degree  of  latitude,  but  as  the  poles  are  approached,  the  de- 
grees of  longitude  diminish  in  length,  because  the  Earth  grows 
smaller  in  circumference  from  the  equator  toward  the  poles ; 
hence  the  lines  surrounding  it  become  less  and  less.     This  will 
be  made  obvious  by  Fig.  259. 

Let  this  figure  represent  the 
Earth,  N  being  the  north  pole, 
S  the  south  pole,  and  E  W  the 
equator.  The  lines  10,  20,  30, 
and  so  on,  are  the  parallels  of 
latitude,  and  the  lines  N  a  S,  N 
b  S,  &c.,  are  meridian  lines,  or 
those  of  longitude. 

The  latitude  of  any  place  on 
the  globe,  is  the  number  of  de- 
grees between  that  place  and 
the  equator,  measured  on  a  me- 
ridian line;  thus,  x  is  irik' lati- 
tude 40  degrees,  because  lie  x  Parallel*  of  Longitude, 
q  part  of  the  meridian  contains 

40  degrees.  The  longitude  of  a  place  is  the  number  of  degrees 
it  is  situated  east  or  west  from  any  meridian  line ;  thus,  v  is  20 
degrees  west  longitude  from  x,  and  #  is  20  degrees  east  longi- 
tude from  v. 

254.  As  the  equator  divides  the  Earth  into  two  equal  parts, 
or  hemispheres,  there  seems  to  be  a  natural  reason  why  the  de- 
grees of  latitude  should  be  reckoned  from  this  great  circle.     But 


253.  What  is  longitude  7  How  many  degrees  of  longitude  are  there,  east  or  west  ? 
What  is  the  latitude  of  any  place  ?  What  is  the  longitude  of  a  place  1  254.  Why  are 
the  degrees  of  latitude  reckoned  from  the  equator?  What  is  said  concerning  the 
places  from  which  the  degrees  of  longitude  have  been  reckoned  1  What  is  the  in- 
convenience of  estimating  longitude  from  a  place  in  each  country  7  From  what 
place  is  longitude  reckoned  in  Europe  and  America  1 


LATITUDE    AND    LONGITUDE.  331 

from  east  to  west  there  is  no  natural  division  of  the  Earth,  each 
meridian  line  being  a  great  circle,  dividing  the  Earth  into  two 
hemispheres,  and  hence  there  is  no  natural  reason  why  longi- 
tude should  be  reckoned  from  one  meridian  any  more  than  an- 
other. It  has,  therefore,  been  customary  for  writers  and  mar- 
iners to  reckon  longitude  from  the  capital  of  their  own  country ; 
as  the  English  from  London,  the  French  from  Paris,  and  the 
Americans  from  Washington.  But  this  mode,  it  is  apparent, 
must  occasion  much  confusion,  since  each  writer  of  a  different 
nation  would  be  obliged  to  correct  the  longitude  of  all  other 
countries,  to  make  it  agree  with  his  own.  More  recently,  there- 
fore, the  writers  of  Europe  and  America  have  selected  the  royal 
observatory,  at  Greenwich,  near  London,  as  the  first  meridian, 
and  on  most  maps  and  charts  lately  published,  longitude  is 
reckoned  from  that  place. 

255.  How  Latitude  is  Found. — The  latitude  of  any  place  is 
determined,  by  taking  the  altitude  of  the  Sun  at  mid-day,  and 
then  subtracting  this  from  90  degrees,  making  proper  allow- 
ances for  the  Sun's  place  in  the  heavens.     The  reason  of  this 
will  be  understood,  when  it  is  considered  that  the  whole  num- 
ber of  degrees  from  the  zenith  to  the  horizon  is  90,  and  there- 
fore if  we  ascertain  the  Sun's  distance  from  the  horizon,  that  is, 
his  altitude,  by  allowing  for  the  Sun's  declination  north  or  south 
of  the  equator,  and  subtracting  this  from  the  whole  number,  the 
latitude  of  the  place  will  be  found.     Thus,  suppose  that  on  the 
20th  of  March,  when  the  Sun  is  at  the  equator,  his  altitude 
from  any  place  north  of  the  equator  should  be  found  to  be  48 
degrees  above  the  horizon ;  this,  subtracted  from  90,  the  whole 
number  of  the  degrees  of  latitude,  leaves  42,  which  will  be  the 
latitude  of  the  place  where  the  observation  was  made. 

256.  If  the  Sun,  at  the  time  of  observation,  has  a  declination 
north  or  south  of  the  equator,  this  declination  must  be  added 
to,  or  subtracted  from,  the  meridian  altitude,  as  the  case  may 
be.     For  instance,  another  observation  being  taken  at  the  place 
where  the  latitude  was  found  to  be  42,  when  the  Sun  had  a 
declination  of  8  degrees  north,  then  his  altitude  would  be  8  de- 
grees greater   than  before,   and  therefore  56,  instead  of  48. 
Now,  subtracting  this  8,  the  Sun's  declination,  from  56,  and  the 
remainder  from  90,  and  the  latitude  of  the  place  will  be  found 
42,  as  before.     If  the  Sun's  declination  be  south  of  the  equator, 

255.  How  is  the  latitude  of  a  place  determined'?  Give  an  example  of  the  method 
of  finding  the  latitude  of  the  same  place  at  different  seasons  of  the  year?  256.  When 
must  the  Sun's  declination  from  the  equator  be  added  to,  and  when  subtracted  from, 
his  meridian  altitude  ? 


332  LATITUDE    AND    LONGITUDE. 

and  the  latitude  of  the  place  north,  his  declination  must  be 
added  to  the  meridian  altitude  instead  of  being  subtracted  from 
it.  The  same  result  may  be  obtained  by  taking  the  meridian 
altitude  of  any  of  the  fixed  stars,  whose  declinations  are  known, 
nstead  of  the  Sun's,  and  proceeding  as  above  directed. 

257.  How  Longitude  is  Found. — There  is  more  difficulty  in 
ascertaining  the  degrees  of  longitude,  than  those  of  latitude, 
because,  as  above  stated,  there  is  no  fixed  point,  like  that  of  the 
equator,  from  which  its  degrees  are  reckoned.     The  degrees  of 
longitude  are  therefore  estimated  from  Greenwich,  and  are  as- 
certained by  the  following  methods  : — 

258.  When  the  Sun  comes  to  the  meridian  of  any  place,  it 
is  noon,  or  12  o'clock,  at  that  place,  and  therefore,  since  the 
equator  is  divided  into  360  equal  parts,  or  degrees,  and  since 
the  Earth  turns  on  its  axis  once  in  24  hours,  15  degrees  of  the 
equator  will  correspond  with  one  hour  of  time,  for  360  degrees 
being  divided  by  24  hours,  will  give  15.     The  Earth,  therefore, 
moves  in  her  daily  revolution,  at  the  rate  of  15  degrees  for 
every  hour  of  time.     Now,  as  the  apparent  course  of  the  Sun  is 
from  east  to  west,  it  is  obvious  that  he  will  come  to  any  me- 
ridian lying  east  of  a  given  place,  sooner  than  to  one  lying  west 
of  that  place,  and  therefore  it  will  be  12  o'clock  to  the  east  of 
any  place,  sooner  than  at  that  place,  or  to  the  west  of  it. 

259.  When,  therefore,  it  is  noon  at  any  one  place,  it  will  be 
1  o'clock  at  all  places  15  degrees  to  the  east  of  it,  because  the 
Sun  was  at  the  meridian  of  such  places  an  hour  before ;  and  so, 
on  the  contrary,  it  will  be  11  o'clock,  15  degrees  west  of  tlie 
same  place,  because  the  Sun  has  still  an  hour  to  travel  before 
he  reaches  the  meridian  of  that  place.     It  makes  no  difference, 
then,  where  the  observer  is  placed,  since,  if  it  is  12  o'clock 
where  he  is-,  it  will  be  1  o'clock  15  degrees  to  the  east  of  him, 
and  11  o'clock  15  degrees  to  the  west  of  him,  and  so  in  this 
proportion,  let  the  time  be  more  or  less.     Now,  if  any  celestial 
phenomenon  should  happen,  such  as  an  eclipse  of  the  Moon,  or 
of  Jupiter's  satellites,  the  difference  of  longitude  between  two 
places  where  it  is  observed,  may  be  determined  by  the  differ- 
ence of  the  times  at  which  it  appeared  to  take  place.     Thus,  if 
the  Moon  enters  the  Earth's  shadow  at  6  o'clock  in  the  evening, 
as  seen  at  Philadelphia,  and  at  half  past  6  o'clock  at  another 

257.  Why  is  there  more  difficulty  in  ascertaining  the  degrees  of  longitude  than  of 
latitude?  258.  How  many  degree's  of  longitude  does  the  surface  of  the  Earth  pass 
through  in  an  hour?  259.  Suppose  it  is  noon  at  any  given  place,  what  o'clock  will 
it  be  fifteen  degrees  to  the  east  of  that  place  1  Explain  the  reason.  How  may  longi- 
tude be  determined  by  an  eclipse? 


FIXED    STARS.  333 

place,  then  this  place  is  half  an  hour,  or  7£  degrees,  to  the  east 
of  Philadelphia,  because  1%  degrees  of  longitude  are  equal  to 
half  an  hour  of  time.  To  apply  these  observations  practically, 
it  is  only  necessary  that  it  should  be  known  exactly  at  what 
time  the  eclipse  takes  place  at  a  given  point  on  the  Earth. 

260.  USE  OF  THE  CHRONOMETER. — Suppose  two  chronome- 
ters, which  are  known  to  go  at  exactly  the  same  rate,  are  made 
to  indicate  12  o'clock  by  the  meridian  line  at  Greenwich,  and 
the  one  be  taken  to  sea,  while  the  other  remains  at  Greenwich. 
Then  suppose  the  captain,  who  takes  his  chronometer  to  sea,  has 
occasion  to  know  his  longitude.     In  the  first  place,  he  ascer- 
tains, by  an  observation  of  the  Sun,  when  it  is  12  o'clock  at 
the  place  where  he  is,  and  then  by  his  time-piece,  when  it  is 
12  o'clock  at  Greenwich,  and  by  allowing  15  degrees  for  every 
hour  of  the  difference  in  time,  he  will  know  his  precise  longi- 
tude in  any  part  of  the  world. 

261.  For  example,  suppose  the  captain  sails  with  his 'chro- 
nometer for  America,  and  after  being  several  weeks  at  sea,  finds 
by  observation  that  it  is  1 2  o'clock  by  the  Sun,  and  at  the  same 
time  finds  by  his  chronometer,  that  it  is  4  o'clock  at  Greenwich. 
Then,  because  it  is  noon  at  his  place  of  observation  after  it  is 
noon  at  Greenwich,  he  knows  that  his  longitude  is  west  from 
Greenwich,  and  by  allowing  15  degrees  for  every  hour  of  the 
difference,  his  longitude  is  ascertained.     Thus,  15  degrees,  mul- 
tiplied by  4  hours,  give   60  degrees  of  west  longitude  from 
Greenwich.     If  it  is  noon  at  the  place  of  observation,  before  it 
is  noon  at  Greenwich,  then  the  captain  knows  that  his  longitude 
is  east,  and  his  true  place  is  found  in  the  same  manner. 


FIXED    STARS. 

262.  The  stars  are  called  fixed,  because  they  have  been  ob- 
served not  to  change  their  places  with  respect  to  each  other. 
They  may  be  distinguished  by  the  naked  eye  from  the  planets 
of  our  system  by  their  scintillations,  or  twinkling.  The  stars 
are  divided  into  classes,  according  to  their  magnitudes,  and  are 
called  stars  of  the  first,  second,  and  so  on  to  the  sixth  magni- 

260.  Explain  the  principles  on  which  longitude  is  determined  by  the  chronometer. 
261.  Suppose  the  captain  finds  by  his  chronometer  that  it  is  12  o'clock  where  he  is 
6  hours  later  than  at  Greenwich,  what  then  would  be  his  longitude?  Suppose  he 
finds  it  to  be  12  o'clock  4  hours  earlier  where  he  is.  than  at  Greenwich,  what  then 
would  be  his  longitude'?  262.  Why  are  the  stars  called  fixed  ?  How  may  the  stars 
be  distinguished  from  the  planets?  The  stars  are  divided  into  classes,  according  to 
their  magnitudes ;  how  many  classes  are  there  ?  How  manv  stars  may  be  seen 
with  the  naked  eye  in  the  whol«  firmament? 


334  FIXED    STARS. 

tude.  About  2,000  stars  may  be  seen  with  the  naked  eye  in 
the  whole  vault  of  the  heavens,  though  only  about  1,000  are 
above  the  horizon  at  the  same  time.  Of  these,  about  1Y  are  of 
the  first  magnitude,  50  of  the  second  magnitude,  and  150  of  the 
third  magnitude.  The  others  are  of  the  fourth,  fifth,  and  sixth 
magnitudes,  the  last  of  which  are  the  smallest  that  can  be  dis- 
tinguished with  the  naked  eye. 

263.  It  might  seem  incredible,  that  on  a  clear  night  only 
about  1,000  stars  are  visible,  when  on  a  single  glance  at  the 
different  parts  of  the  firmament,  their  numbers  appear  innumer- 
able.    But  this  deception  arises  from  the  confused  and  hasty 
manner  in  which  they  are  viewed ;  for  if  we  look  steadily  on  a 
particular  portion  of  sky,  and  count  the  stars  contained  within 
certain  limits,  we  shall  be  surprised  to  find  their  number  so  few. 

264.  The  nearest  fixed  stare  to  our  system,  from  the  most 
accurate   astronomical   calculations,   can   not  be   nearer  than 
20,000,000,000,000,  or  20  trillions  of  miles  from  the  Earth,  a 
distance  so  immense,  that  light  can  not  pass  through^  it  in  less 
than  three  years.     Hence,  were  these  stars  annihilated  at  the 
present  time,  their  light  would  continue  to  flow  toward  us,  and 
they  would  appear  to  be  in  the  same  situation  to  us,  three  years 
hence,  that  they  do  now. 

265.  Our  Sun,  seen  from  the  distance  of  the  nearest  fixed 
stars,  would  appear  no  larger  than  a  star  of  the  first  magnitude 
does  to  us.     These  stars  appear  no  larger  to  us,  when  the  Earth 
is  in  that  part  of  her  orbit  nearest  to  them,  than  they  do,  when 
she  is  in  the  opposite  part  of  her  orbit ;  and  as  our  distance 
from  the  Sun  is  95,000,000  of  miles,  we  must  be  twice  this 
distance,  or  the  whole  diameter  of  the  Earth's  orbit,  nearer  a 
given  fixed  star  at  one  period  of  the  year  than  at  another.     The 
difference,  therefore,  of  190,000,000  of  miles,  bears  so  small  a 
proportion  to  the  whole  distance  between  us  and  the  fixed  stars, 
as  to  make  no  appreciable  difference  in  their  sizes,  even  when 
assisted  by  the  most  powerful  telescopes. 

266.  The  amazing  distances  of  the  fixed  stars  may  also  be 
inferred  from  the  return  of  comets  to  our  system,  after  an  ab- 
sence of  several  hundred  years. 

The  velocity  with  which  some  of  these  bodies  move,  when 

2C3.  Why  does  there  appear  to  be  more  stars  than  there  really  are  ?  264.  What  is 
the  computed  distance  of  the  nearest  fixed  stars  from  the  Earth  1  How  long  would 
it  take  liiiht  to  reach  us  from  the  fixed  stars?  265.  How  large  would  our  Sun  ap- 

§ear  at  the  distance  of  the  fixed  stars?    What  is  said  concerning  the  difference  of  the 
istance  between  the  Earth  and  the  fixed  stars  at.  different  seasons  of  the  year,  and 
of  their  different  appearance  in  consequence?    266.  How  may  the  distances  of  the 
fixed  stars  be  inferred,  by  the  long  absence  and  return  of  comets  1 


COMETS.  335 

nearest  the  Sun,  has  been  computed  at  nearly  a  million  of  miles 
in  an  hour,  and  although  their  velocities  must  be  perpetually 
retarded,  as  they  recede  from  the  Sun,  still,  in  250  years  of 
time,  they  must  move  through  a  space  which  to  us  would  be 
infinite.  The  periodical  return  of  one  comet  is  known  to  be 
upward  of  500  years,  making  more  than  250  years  in  perform- 
ing its  journey  to  the  most  remote  part  of  its  orbit,  and  as 
many  in  returning  back  to  our  system ;  and  that  it  must  still 
always  be  nearer  our  system  than  the  fixed  stars,  is  proved  by 
its  return ;  for  by  the  laws  of  gravitation,  did  it  approach  nearer 
another  system  it  would  never  again  return  to  ours. 

267.  From  such  proofs  of  the  vast  distances  of  the  fixed  stars, 
there  can  be  no  doubt  that  they  shine  with  their  own  light, 
like  our  Sun,  and  hence  the  conclusion  that  they  are  Suns  to 
other  worlds,  which  move  around  them,  as  the  planets  do  around 
our  Sun.     Their   distances    will,    however,   prevent   our   ever 
knowing,  except  by  conjecture,  whether  this  is  the  case  or  not, 
since,  were  they  millions  of  times  nearer  us  than  they  are,  we 
should  not  be  able  to  discover  the  reflected  light  of  their  planets, 

COMETS. 

268.  Besides  the  planets,  which  move  round  the  Sim  in  reg- 
ular order  and  in  nearly  circular  orbits,  there  belongs  to  th& 
solar  system  an  unknown  number  of  bodies  called  Comets,  which 
move  round  the  Sun  in  orbits  exceedingly  eccentric,  or  elliptical, 
and  whose  appearance  among  our  heavenly  bodies  is  only  occa- 
sional.    Comets,  to  the  naked  eye,  have  no  visible  disc,  but 
shine  with  a  faint,  glimmering  light,  and  are  accompanied  by  a 
train  or  tail,  turned  from  the  Sun,  and  which  is  sometimes  of 
immense  length.     They  appear  in  every  region  of  the  heavens^ 
and  move  in  every  possible  direction. 

269.  Number   and  Periods  of   Comets. — The   number   of 
comets  is  unknown,  though  some  astronomers  suppose  that 
there  are  nearly  500  belonging  to  our  system.     Ferguson,  who 
wrote  in  about  1760,  supposed  that  there  were  less  than  30 
comets  which  made  us  occasional  visits ;  but  since  that  period 
the  elements  of  the  orbits  of  nearly  100  of  these  bodies  have 
l>een  computed. 

Of  these,  however,  there  are  only  three  whose  periods  of  re- 
turn among  us  are  known  with  any  degree  of  certainty.     The 

267.  On  what  grounds  is  it  supposed  that  the  fixed  stars  are  suns  to  other  worlds! 
269.  What  number  of  comets  are  supposed  to  belong  to  our  system  1 


336  COMETS. 

first  of  these  has  a  period  „„„„! I?;,f!!: 

of  75  years ;  the  second  a 
period  of  129  years;  and 
the  third  a  period  of  575 
years.  The  third  appeared 
in  1680,  and  therefore  can 
hot  be  expected  again  until 
the  year  2225.  This 
comet,  Fig.  260,  in  1680, 
excited  the  most  intense 
interest  among  the  astronomers  of  Europe,  on  account  of  its 
great  apparent  size  and  near  approach  to  our  system.  In  the 
most  remote  part  of  its  orbit,  its  distance  from  the  Sun  was  es- 
timated at  about  11,200,000,000  of  miles.  At  its  nearest  ap- 
proach to  the  Sun,  which  was  only  about  50,000  miles,  its 
velocity,  according  to  Sir  Isaac  Newton,  was  880,000  miles  in 
an  hour ;  and  supposing  it  to  have  retained  the  Sun's  heat,  like 
other  solid  bodies,  its  temperature  must  have  been  about  2,000 
times  that  of  red  hot  iron.  The  tail  of  this  comet  was  at  least 
100,000,000  of  miles  long. 

270.  In  the  Edinburgh  Encyclopedia,  article  Astronomy,  there 
is  the  most  complete  table  of  comets  yet  published.     This  table 
contains  the  elements  of  97  comets,  calculated  by  different  as 
tronomers,  down  to  the  year  1808. 

From  this  table  it  appears  that  24  comets  have  passed  be- 
tween the  Sun  and  the  orbit  of  Mercury ;  33  between  the  orbits 
of  Venus  and  the  Earth;  15  between  the  orbits  of  the  Earth 
and  Mars ;  3  between  the  orbits  of  Mars  and  Ceres ;  and  1  be- 
tween the  orbits  of  Ceres  and  Jupiter.  It  also  appears  by  this 
table  that  49  comets  have  moved  round  the  Sun  from  west  to 
east,  and  48  from  east  to  west. 

271.  Nature  of  Comets. — Of  the  nature  of  these  wandering 
planets  very  little  is  known.     When  examined  by  a  telescope, 
they  appear  like  a  mass  of  vapors  surrounding  a  dark  nucleus. 
When  the  comet  is  at  its  perihelion,  or  nearest  the  Sun,  its  coloi 
seems  to  be  heightened  by  the  intense  light  or  heat  of  that 
luminary,  and  it  then  often  shines  with  more  brilliancy  than 
the  planets.     At  this  time  the  tail  or  train,  which  is  always 
directly  opposite  to  the  Sun,  appears  at  its  greatest  length,  but 
is  commonly  so  transparent  as  to  permit  the  fixed  stars  to  be 


270.  How  many  have  had  the  elements  of  their  orbits  estimated  by  astronomers? 
How  many  are  there  whose  periods  of  return  are  known?  271.  What  is  said  of  the 
tomet  of  16SO? 


PARALLAX. 


837 


seen  through  it.  A  variety  of  opinions  have  been  advanced  by 
astronomers  concerning  the  nature  and  causes  of  these  trains, 
but  no  satisfactory  theory  has  been  offered. 

A  new  comet  was  discovered  by  Miss  Maria  Mitchell,  of 
Kantucket,  in  October,  1847,  for  which  she  received  the  gold 
medal  of  the  king  of  Denmark,  offered  for  the  first  discovery  of 
a  new  cornet  in  any  country. 

PARALLAX. 

272.  Parallax  is  the  difference  between  the  true  and  apparent 
place  of  a  celestial  body.  The  apparent  place  is  that  in  which 
the  body  seems  to  be  when  viewed  fror  i  the  surface  of  the 
Earth,  the  true  place  being  that  in  which  it  would  appear  if 
seen  from  the  center  of  the  earth. 

This  will  be  understood 
by  Fig.  261,  where  if  we  FIG 

suppose  a  spectator  placed 
at  G,  in  the  Earth's  center, 
he  would  see  the  moon  E, 
among  the  stars  at  I,  whereas 
without  changing  the  posi- 
tion of  the  moon,  if  that 
body  is  seen  from  A,  on  the 
surface  of  the  Earth,  it 
would  appear  among  the 
stars  at  K.  Now  I  is  the 
true  and  K  the  apparent 
place  of  the  moon,  the  space 
between  them,  being  the 
Moon's  parallax. 

The  parallax  of  a  body  is  greatest  when  on  the  sensible  hor- 
izon, (170,)  or  at  the  moment  when  it  becomes  visible  to  the 
eye.  From  this  point  it  diminishes  until  it  reaches  the  zenith, 
or  the  highest  place  in  the  heavens,  when  its  parallax  ceases 
entirely.  Thus  it  will  be  seen  by  the  figure,  that  the  parallax 
of  the  moon  is  less  when  at  D,  than  it  was  at  E,  and  that  when 
it  arrives  at  the  zenith,  Z,  its  position  is  the  same  whether  seen 
from  the  center  of  the  Earth,  G,  or  from  its  surface,  A. 

The  greater  the  distance  of  the  heavenly  body  from  the  spec- 
tator, the  less  is  its  parallax. 


Diurnal  Parallax. 


272.  What  is  parallax  ?  What  is  the  apparent  place  of  a  celestial  body  ?  What  \* 
the  true  place  of  such  a  body  ?  Explain  Fig.  261,  and  show  why  there  is  no  parallax 
when  the  body  is  in  the  zenith  ? 


15 


338  PARALLAX. 

Thus  were  the  Moon  at  e  instead  of  at  E,  her  parallax  would 
be  only  equal  to  p  K,  instead  of  I  K.  Hence  the  Moon,  being 
the  nearest  celestial  body,  has  the  greatest  parallax,  the  differ- 
ence of  her  place  among  the  stars,  when  seen  from  the  surface 
of  the  Earth,  A,  and  the  center  G,  being  about  4,000  miles. 

273.  Parallax  of  the  Stars. — The  stars  are  at  such  immense 
distances  from  the  Earth,  that  the  difference  of  station  between 
the  center  and  surface  of  the  Earth  makes  no  perceptible  change 
in  their  places,  and  hence  they  have  no  parallax. 

274.  Diurnal  Parallax. — This  applies  to  the  solar  system, 
and  takes  place  every  day  in  the  apparent  rotation  of  the  planets 
around  the  Earth.     The  Moon,  as  above  shown,  has  a  parallax 
when  she  rises,  which  diminishes  until  she  reaches  the  zenith, 
when  it  ceases  entirely ;  the  same  is  the  case  with  the  Sun  and 
planets,  which  have  sensible  parallaxes. 

275.  Annual  Parallax. — This  is  the  difference  in  the  appa- 
rent places  of  the  celestial  bodies,  as  seen  from  the  Earth  at  the 
opposite  points  of  her  orbit,  during  her  annual  revolution  round 
the  Sun. 

Suppose  A,  Fig.  262, 
to  be  a  stationary  ce- 
lestial object,  then  as 
the  Earth  makes  her  an- 
nual revolution  around 
the  Sun  S,  this  object 
at  one  time  will  appear 
among  the  stars  at  E, 
but  six  months  after, 
when  the  Earth  comes 

to    the    Opposite    point  Annual  Parallax. 

in  her  orbit,  the  same 

object  will  be  seen  at  C,  the  space  from  C  to  E  being  the  an- 
nual parallax  of  the  object  A.  But  the  distances  of  the  stars 
are  so  great  that  the  diameter  of  the  Earth's  orbit,  or  190,000,000 
of  miles  make  no  difference  in  their  apparent  places.  Were  the 
fixed  stars  within  19,000,000,000,000,  or  19  trillions  of  miles, 
their  distance  could  be  told  by  their  parallaxes. 

But  since,  as  above  stated,  these  celestial  points  have  no  sens- 
ible parallaxes,  their  distances  must  be  greater  than  this,  but 
how  much  is  unknown. 


273.  Why  have  the  stars  no  parallaxes  ?    274.  What  is  diurnal  parallax  1   275.  What 
is  annual  parallax') 


ELECTRICITY.  339 


CHAPTER    XIII. 

ELECTRICITY. 

276.  THE  science  of  Electricity,  which  now  ranks  as  an  im- 
portant branch  of  Natural  Philosophy,  is  wholly  of  modern  date. 
The  ancients  were  acquainted  with  a  few  detached  facts  de- 
pendent on  the  agency  of  electrical  influence,  but  they  never 
imagined  that  it  was  extensively  concerned  in  the  operations  of 
nature,  or  that  it  pervaded  material  substances  generally.     The 
term  electricity  is  derived  from  electron,  the  Greek  name  of 
amber,  because  it  was  known  to  the  ancients,  that  when  that 
substance  was  rubbed  or  excited,  it  attracted  or  repelled  small 
light  bodies,  but  it  was  then  unknown  that  other  substances 
when  excited,  would  do  the  same. 

When  a  piece  of  glass,  sealing-wax,  or  amber,  is  rubbed  with 
a  dry  hand,  and  held  toward  small  and  light  bodies,  such  as 
threads,  hairs,  feathers,  or  straws,  these  bodies  will  fly  toward 
the  surface  thus  rubbed,  and  adhere  to  it  for  a  short  time.  The 
influence  by  which  these  small  substances  are  drawn,  is  called 
electrical  attraction  ;  the  surface  having  this  attractive  powei 
is  said  to  be  excited  ;  and  the  substances  susceptible  of  this  ex- 
citation, are  called  electrics.  Substances  not  having  this  attrac- 
tive power  when  rubbed,  are  called  non-electrics. 

277.  The  principal  electrics  are  amber,  resin,  sulphur,  glass, 
the  precious  stones,  sealing-wax,  and  the  fur  of  quadrupeds. 
But  the  metals,  and  many  other  bodies,  may  be  excited  when 
insulated  and  treated  in  a  certain  manner. 

After  the  light  substances  which  had  been  attracted  by  the 
excited  surface,  have  remained  in  contact  with  it  a  short  time, 
the  force  which  brought  them  together  ceases  to  act,  or  acts  in 
a  contrary  direction,  and  the  light  bodies  are  repelled,  or  thrown 
away  from  the  excited  surface.  Two  bodies,  also,  which  have 
been  in  contact  with  the  excited  surface,  mutually  repel  each 
other. 

278.  Electroscope. — Various  modes  have  been  devised   for 
exhibiting  distinctly  the  attractive  and  repulsive  agencies  of 

276.  From  what  is  the  term  electricity  derived  1  What  is  electrical  attraction1? 
What  are  electrics  7  What  are  non-electrics'?  277.  What  are  the  principal  dec- 
tries  1  What  is  meant  by  electrical  repulsion  ?  278.  What  is  an  electroscope  1 


340 


ELECTRICITY. 


electricity,  and  for  obtaining  indications  of  its  presence,  when  it 
exists  only  in  a  feeble  degree.  Instruments  for  this  purpose  are 
termed  Electroscopes. 

One  of  the  simplest  instruments  of  this  kind  consists  of  a  me- 
tallic needle,  terminated  at  each  end  by  a  light  pith-ball,  which 
is  covered  with  gold  leaf,  and  supported  horizontally  at  its  center 
by  a  fine  point,  Fig.  263.  When  a  stick  of  sealing-wax,  or  a 
glass  tube,  is  excited,  and  then  presented  to  one  of  these  balls, 
the  motion  of  the  needle  on  its  pivot  will  indicate  the  electrical 
influence. 


FIG.  263. 


FIG.  264. 


Electroscope. 


Electrical  Attraction. 


279.  If  an  excited  substance  be  brought  near  a  ball  made  of 
pith,  or  cork,  suspended  by  a  silk  thread,  the  ball  will,  in  the 
first  place,  approach  the  electric,  as  at  A,  Fig.  264,  indicating 
an  attraction  toward  it,  and  if  the  position  of  the  electric  will 
allow,  the  ball  will  come  into  contact  with  the  electric,  and  ad- 
here to  it  for  a  short  time,  and  will  then  recede  from  it,  show- 
ing that  it  is  repelled,  as  at  B.     If,  now,  the  ball  which  had 
touched  the  electric,  be  brought  near  another  ball,  which  has 
had  no  communication  with  an  excited  substance,  these  two 
balls  will  attract  each  other,  and  come  into  contact ;  after  which 
they  will  repel  each  other,  as  in  the  former  case. 

It  appears,  therefore,  that  the  excited  body,  as  the  stick  of 
sealing-wax,  imparts  a  portion  of  its  electricity  to  the  ball,  and 
that  when  the  ball  is  also  electrified,  a  mutual  repulsion  then 
takes  place  between  them.  Afterwards,  the  ball,  being  electri- 
fied by  contact  with  the  electric,  when  brought  near  another 
ball  not  electrified,  transfers  a  part  of  its  electrical  influence  to 
that,  after  which  these  two  balls  repel  each  other,  as  in  the 
former  instance. 

280.  Thus,  when  one  substance  has  a  greater  or  less  quan- 

279.  When  do  two  electrified  bodies  attract,  and  when  do  they  repel  each  other? 
280.  How  will  two  bodies  act,  one  having  more,  and  the  other  less,  than  the  natural 
quantity  of  electricity,  when  brought  near  each  other  1  How  will  they  act  when 
both  have  more  or  Use  than  their  natural  quantity  1 


841 

tity  of  electricity  than  another,  it  will  attract  the  other  sub- 
stance, and  when  they  are  in  contact  will  impart  to  it  a  portion 
of  this  superabundance ;  but  when  they  are  both  equally  elec- 
trified, both  having  more  or  less  than  their  natural  quantity  of 
electricity,  they  will  repel  each  other. 

ELECTRICAL  THEORIES. — To  account  for  these  phenomena, 
two  theories  have  been  advanced,  one  by  Dr.  Franklin,  who 
supposes  there  is  only  one  electrical  fluid,  and  the  other  by 
Du  Fay,  who  supposes  that  there  are  two  distinct  fluids. 

281.  Franklin's  Theory. — Dr.   Franklin  supposed  that  all 
terrestrial  substances  were  pervaded  with  the  electrical  fluid,  and 
that  by  exciting,  an  electric,  the  equilibrium  of  this  fluid  was 
destroyed,  so  that  one  part  of  the  excited  body  contained  more 
than  its  natural  quantity  of  electricity,  and  the  other  part  less. 
If  in  this  state  a  conductor  of  electricity,  as  a  piece  of  metal,  be 
brought  near  the  excited  part,  the  accumulated  electricity  would 
be  imparted  to  it,  and  then  this  conductor  would  receive  more 
than  its  natural  quantity  of  the  electric  fluid.     This  he  called 
positive  electricity.     But  if  a  conductor  be  connected  with  that 
part  which  has  less  than  its  ordinary  share  of  the  fluid,  then 
the  conductor  parts  with  a  share  of  its  own,  and  therefore  will 
then  contain  less  than  its  natural  quantity.     This  he  called 
negative  electricity.     When  one  body  positively  and  another 
negatively  electrified,  are  connected  by  a  conducting  substance, 
the  fluid  rushes  from  the  positive  to  the  negative  body,  and  the 
equilibrium  is  restored.     Thus,  bodies  which  are  said  to  be  pos- 
itively electrified,  contain  more  than  their  natural  quantity  of 
electricity,  while  those  which  are  negatively  electrified,  contain 
less  than  their  natural  quantity. 

282.  Du  Fay's  Theory. — The  other  theory  is  explained  thus. 
When  a  piece  of  glass  is  excited  and  made  to  touch  a  pith-ball, 
as  above  stated,  then  that  ball  will  attract  another  ball,  after 
which  they  will  mutually  repel  each  other,  and  the  same  will 
happen  if  a  piece  of  sealing-wax  be  used  instead  of  the  glass. 
But  if  a  piece  of  excited  glass,  and  another  of  wax,  be  made  to 
touch  two  separate  balls,  they  will  attract  each  other ;  that  is, 
the  ball  which  received  its  electricity  from  the  wax  will  attract 
that  which  received  its  electricity  from  the  glass,  and  will  be 

281.  Explain  Dr.  Franklin's  theory  of  electricity.  What  is  meant  by  positive,  and 
wnat  by  negative  electricity  ?  What  is  the  consequence,  when  a  positive  and  a  neg- 
ative body  are  connected  by  a  conductor?  282.  Explain  Du  Fay's  theory.  When 
two  balls  are  electrified,  one  with  glass  and  the  other  with  wax,  will  they  attract  or 
repel  each  other  1  What  are  the  two  electricities  called  7  From  what  substances 
are  the  two  electricities  obtained? 


942 


ELECTRICITY. 


attracted  by  it.  Hence  Du  Fay  concludes  that  electricity  con- 
sists of  two  distinct  fluids,  which  exist  together  in  all  bodies — 
that  they  have  a  mutual  attraction  for  each  other — that  they 
are  separated  by  the  excitation  of  electrics,  and  that  when  thus 
separated,  and  transferred  to  non-electrics,  as  to  the  pith-balls, 
their  mutual  attraction  causes  the  balls  to  rush  toward  each 
other.  These  two  principles  he  called  vitreous  and  resinous 
electricity.  The  vitreous  being  obtained  from  glass,  and  the 
resinous  from  wax  and  other  resinous  substances. 

Dr.  Franklin's  theory  is  by  far  the  most  simple,  and  will  ac- 
count for  most  of  the  electrical  phenomena  equally  well  with 
that  of  Du  Fay,  and  therefore  has  been  adopted  by  the  most 
able  and  recent  electricians. 

283.  It  is  found  that  some  substances  conduct  the  electric 
fluid  from  a  positive  to  a  negative  surface  with  great  facility, 
while  others  conduct  it  with  difficulty,  and  others  not  at  all. 
Substances  of  the  first  kind  are  called  conductors,  and  those  of 
the  last  non-conductors.     The  electrics,  or  such  substances  as 
being  excited,  communicate  electricity,  are  all  non-conductors, 
while  the  non-electrics,  or  such  substances  as  do  not  communi- 
cate electricity  on  being  merely  excited,  are  conductors.     The 
conductors  are  the  metals,  charcoal,  water,  and  other  fluids,  ex- 
cept the  oils ;  also  smoke,  steam,  ice,  and  snow.     The  best  con- 
ductors are  gold,  silver,  platina,  brass,  and  iron. 

The  electrics,  or  non-conductors,  are  glass,  amber,  sulphur, 
resin,  wax,  silk,  most  hard  stones,  and  the  furs  of  some  animals. 

A  body  is  said  to  be  insulated,  when  it  is  supported  or  sur- 
rounded by  an  electric.  Thus,  a  stool  standing  on  glass  legs, 
is  insulated,  and  a  plate  of  metal  laid  on  a  plate  of  glass,  is 
insulated.  • 

284.  Electrical  Machines. — When  large    quantities  of  the 
electric  fluid  are  wanted  for  experiment,  or  for  other  purposes, 
it  is  procured  by  an  electrical  machine.     These  machines  are  of 
various  forms,  but  all  consist  of  an  electric  substance  of  consid- 
erable dimensions ;  the  rubber  by  which  this  is  excited ;  the 
prime  conductor,  on  which  the  electric  matter  is  accumulated ; 
the  insulator,  which  prevents  the  fluid  from  escaping ;  and  ma- 
chinery, by  which  the  electric  is  set  in  motion. 

Formerly  a  glass  cylinder  was  employed  as  an  electric,  but 


283.  What  are  conductors?  What  are  non-conductors 7  What  substances  are 
conductors  1  What  substances  are  the  best  conductors  1  What  substances  are  elec- 
trics, or  non-conductors?  When  is  a  body  eaid  to  be  insulated?  284.  What  are 
the  several  parts  of  an  electrical  machine  ? 


ELECTRICITY.  843 

more  recently,  round,  flat  plates  of  glass,  called  plate  machines, 
are  used  instead  of  cylinders.  This  is  a  great  improvement, 
since  both  sides  of  the  plate  are  exposed  to  electrical  friction, 
while  in  the  cylinder,  the  outside  only  could  be  excited. 

This  machine  is  represented  by  Fig.  265,  and  consists  of  a 
circular  plate  of  glass,  from  one  to  two  or  three  feet  in  diameter, 
turning  on  an  axis  of  wood  which  passes  through  its  center. 
The  plate  is  rubbed  as  it  revolves,  by  two  leather  cushions,  A 


FIG.  265. 


Plate  Electrical  Machine. 

and  B,  fixed  at  opposite  points  of  its  circumference,  and  by 
means  of  elastic  slips  of  wood  adjusted  by  screws,  made  to  press 
on  its  surface.  On  the  opposite  side  are  two  other  cushions 
not  seen,  the  plate  revolving  between  them.  A  hollow  brass 
prime  conductor,  C,  supported  by  a  glass  standard  D,  is  attached 
to  the  frame  of  the  machine.  On  each  side  of  the  conductor 
are  branches  of  the  same  metal,  at  the  ends  of  which  are  sharp 
wires  nearly  touching  the  glass  plate,  and  by  means  of  which, 
the  electric  fluid  is  collected  and  conveyed  to  the  conductor. 

285.  Mode  of  Action. — The  manner  in  which  this  machine 
acts  is  easily  understood.  The  friction  of  the  cushions  against 
the  glass  plate,  transfers  the  electrical  fluid  from  the  cushions 
to  the  glass,  so  that  while  the  glass  becomes  positive,  the  cush- 
ions become  negative.  Meantime,  the  fluid,  which  adheres  to 
the  surface  of  the  glass,  is  attracted  by  the  metallic  points  and 

Describe  the  electrical  machine,  Fig.  265.  285.  Whence  comes  the  electricity,  when 
the  plate  is  turned  1  Why  is  it  necessary  to  throw  the  chain  on  the  ground  to  obtain 
more  electricity  1 


344  ELECTRICITY. 

conveyed  to  the  prime  conductor,  which  being  insulated  by  the 
glass  standard,  the  electricity  is  there  accumulated  in  quantities 
proportionate  to  the  surface  of  the  conductor. 

If  the  cushions  are  insulated,  the  quantity  of  electricity  ob- 
tained is  limited,  consisting  of  that,  merely,  which  the  cushions 
contained,  and  when  this  is  transferred  to  the  plate,  no  more 
can  be  obtained.  It  is  then  necessary  to  make  the  cushions 
communicate  with  the  ground,  the  great  reservoir  of  electricity, 
by  laying  the  chain  attached  to  the  cushions  on  the  floor  or 
table,  when  on  again  turning  the  machine,  more  of  the  fluid 
will  be  conveyed  to  the  conductor. 

286.  If  a  person  who  is  insulated  takes  the  chain  in  his 
hand,*the  electric  fluid  will  be  drawn  from  him,  along  the  chain, 
to  the  cushion,  and  from  the  cushion  will  be  transferred  to  the 
prime  conductor,  and  thus  the  person  will  become  negatively 
electrified.     If,  then,  another  person,  standing  on  the  floor,  hold 
his  knuckle  near  him  who  is  insulated,  a  spark  of  electric  fire 
will  pass  between  them,  with  a  crackling  noise,  and  the  equili- 
brium will  be  restored  ;  that  is,  the  electric  fluid  will  pass  from 
him  who  stands  on  the  floor,  to  him  who  stands  on  the  stool. 
But  if  the  insulated  person  takes  hold  of  a  chain,  connected 
with  the  prime  conductor,  he  may  be  considered  as  forming  a 
part  of  the^conductor,  and  therefore  the  electric  fluid  will  be 
accumulated  all  over  his  surface,  and  he  will  be  positively  elec- 
trified, or  will  obtain  more  than  his  natural  quantity  of  electricity. 
If  now  a  person  standing  on  the  floor  touch  this  person,  he  will 
receive  a  spark  of  electrical  fire  from  him,  and  the  equilibrium 
will  again  be  restored. 

287.  If  two  persons  stand  on  two  insulated  stools,  or  if  they 
both  stand  on  a  plate  of  glass,  or  a  cake  of  wax,  the  one  person 
being  connected  by  the  chain  with  the  prime  conductor,  and 
the  other  with  the  cushion,  then,  after  working  the  machine,  if 
they  touch  each  other,  a  much  stronger  shock  will  be  felt  than 
in  either  of  the  other  cases,  because  the  difference  between  their 
electrical  states  will  be  greater,  the  one  having  more  and  the 
other  less  than  his  natural  quantity  of  electricity.     But  if  the 
two  insulated  persons  both  take  hold  of  the  chain  connected 
with  the  prime  conductor,  or  with  that  connected  with  the 

286.  If  an  insulated  person  takes  the  chain,  connected  with  the  cushion,  in  his 
nand,  what  change  will  be  produced  in  his  natural  quantity  of  electricity  1  If  the  in- 
eulatod  person  takes  hold  of  the  chain  connected  with  the  prime  conductor,  and  the 
machine  be  worked,  what  then  will  be  the  change  produced  in  his  electrical  state  7 
387.  If  two  insulated  persons  take  hold  of  the  two  chains,  one  connected  with  the 
prime  conductor,  and  the  other  with  the  cushion,  what  changes  will  be  produced  7 
If  an  insulated  person  takes  the  chain,  what  effect  will  it  produce  on  him  1 


ELECTRICITY. 


345 


cushion,  no  spark  will  pass  between  them,  on  touching  each 
other,  because  they  will  then  both  be  in  the  same  electrical 
state. 

288.  We  have  seen,  Fig.  264,  that  the  pith-ball  is  first  at- 
tracted and  then  repelled,  by  the  excited  electric,  and  that  the 
ball  so  repelled  will  attract,  or  be  attracted  by  other  substances 
in  its  vicinity,  in  consequence  of  having  received  from  the  ex- 
cited body  more  than  its  ordinary  quantity  of  electricity. 

These  alternate  movements  are  amusingly  exhibited  by  plac- 
ing some  small  light  bodies,  such  as  the  figures  of  men  and 
women,  made  of  pith,  or  paper,  between  two  metallic  plates,  the 
one  placed  over  the  other,  as  in  Fig.  266,  the  upper  plate  com- 
municating with  the  prime  conductor,  and  the  other  with  the 
ground.  When  the  electricity  is  communicated  to  the  upper 
plate,  the  little  figures,  being  attracted  by  the  electricity,  will 
jump  up  and  strike  their  heads  against  it,  and  having  received 
a  portion  of  the  fluid,  are  instantly  repelled,  and  again  attracted 
by  the  lower  plate,  to  which  they  impart  their  electricity,  and 


FIG.  266. 


FIG.  267, 


Attraction  and  Repulsion. 


then  are  again  attracted,  and  so  fetch  and  carry  the  electric 
fluid  from  one  to  the  other,  as  long  as  the  upper  plate  contains 
more  than  the  lower  one.  In  the  same  manner,  a  tumbler,  if 
electrified  on  the  inside,  and  placed  over  light  substances,  as 
pith-balls,  will  cause  them  to  dance  for  a  considerable  time. 

288.  Explain  the  reason  why  the  little  images  dance  between  the  two  metallic 
plates,  Fig.  206. 

15*  ' 


846  ELECTRICITY. 

289.  Electrometer. — Instruments  designed  to  measure'  the 
intensity  of  electric  action,  are  called  Electrometers.     Such  an 
instrument  is  represented  by  Fig.  267.     It  consists  of  a  slender 
rod  of  light  wood,  A,  terminated  by  a  pith-ball,  which  serves  as 
an  index.     This  is  suspended  at  the  upper  part  of  the  wooden 
stem,  B,  so  as  to  play  easily  backward  and  forward.     The  ivory 
semicircle  C,  is  affixed  to  the  stem,  having  its  center  coinciding 
with  the  axis  of  motion  of  the  rod,  so  as  to  measure  the  angle 
of  deviation  from  the  perpendicular,  which  the  repulsion  of  the 
ball  from  the  stem  produces  on  the  index. 

When  this  instrument  is  used,  the  lower  end  of  the  stem  is 
set  into  an  aperture  in  the  prime  conductor,  and  the  intensity 
of  the  electric  action  is  indicated  by  the  number  of  degrees  the 
index  is  repelled  from  the  perpendicular. 

.The  passage  of  the  electric  fluid  through  a  perfect  conductor 
is  never  attended  with  light,  or  the  crackling  noise  which  is 
heard  when  it  is  transmitted  through  the  air,  or  along  the  sur- 
face of  an  electric. 

290.  Several  curious  experiments  illustrate  this  principle,  for 
if  fragments  of  tin  foil,  or  other  metal,  be  pasted  on  a  piece  of 
glass,  so  near  each  other  that  the  electric  fluid  can  pass  between 
them,  the  whole  line  thus  formed  with  the  pieces-  of  metal,  will 
be  illuminated  by  the  passage  of  the  electricity  from  one  to  the 
other. 

FIG.  268. 


Franklin. 


In  this  manner  figures  or  words  may  be  formed,  as  in  Fig. 
268,  which,  by  connecting  one  of  its  ends  with  the  prime  con- 
ductor, and  the  other  with  the  ground,  will,  when  the  electric 
fluid  is  passed  through  the  whole,  in  the  dark,  appear  one  con- 
tinuous and  vivid  line  of  fire. 


289.  What  is  an  electrometer  ?  Describe  that  represented  in  Fig.  267,  together 
with  the  mode  of  using  it.  When  the  electric  fluid  passes  along  a  perfect  conductor 
is  it  attended  with  light  or  not  ?  When  it  passes  along  an  electric,  or  through  the 
air,  what  phenomena  does  it  exhibit?  290.  Describe  the  experiment,  Fig.  268,  in- 
tended to  illustrate  this  principle. 


5PrLI2& 

ELECTRICITY.          J7      V-V        OF   TH^47 

291.  Electrical  Light.— Electrical  ligfct  seems  not  to  differ 
in  any  respect,  from  the  light  of  the.  Sun.  or  of  a  burning  lamp. 
Dr.  Wollaston  observed,  that  when  this  light  was  seen  through 
a  prism,  the  ordinary,  colors  arising  from  the  decomposition  of 
light  were  obvious. 

292.  When  the  electric  fluid  is  discharged  from  a  point,  it  is 
always  accompanied  by  a  current  of  air,  whether  the  electricity 
be  positive  or  negative.     The  reason  of  this  appears  to  be,  that 
the  instant  a  particle  of  air  becomes  electrified,  it  repels,  and  is 
repelled,  by  the  point  from  which  it  received  tie  electricity. 

Several  curious  little    experiments   are 
made  on  this   principle.     Thus,  let   two  FIG-  269< 

cross  wires,  as  in  Fig.  269,  be  suspended 
on  a  pivot,  each  having  its  point  bent  in  a 
contrary  direction,  and  electrified  by  being 
placed  on  the  prime  conductor  of  a  ma- 
chine.   These  points,  so  long  as  the  machine 
is  in  action,  will  give  oft'  streams  of  elec- 
tricity ;  and  as  the  particles  of  air  repel  the 
points  by  which  they  are  electrified,  the 
little  machine  will  turn  round  rapidly,  in  the  direction  contrary 
to  that  of  the  stream  of  electricity.     Perhaps,  also,  the*eaction 
of  the  atmosphere  against  the  current  of  air  given  off  by  the 
points,  assists  in  giving  it  motion. 

293.  Leyden  Vials. — When  one  part  or  side  of  an  electric  is 
positively,  the  other  part  or  side  is  negatively  electrified.     Thus, 
if  a  plate  of  glass  be  positively  electrified  on  one  side,  it  will  be 
negatively  electrified  on  the  other,  and  if  the  inside  of  a  glass 
vessel  be  positive,  the  outside  will  be  negative. 

Advantage  of  this  circumstance  is  taken,  in  the  construction 
of  electrical  jars,  called  from  the  place  where  they  were  first 
made,  Leyden  vials. 

The  most  common  form  of  this  jar  is  represented  by  Fig. 
270.  It  consists  of  a  glass  vessel,  coated  on  both  sides  up  to 
A,  with  tin  foil ;  the  upper  part  being  left  naked,  so  as  to  pre- 
vent a  spontaneous  discharge,  or  the  passage  of  the  electric  fluid 
from  one  coating  to  the  other.  A  metallic  rod,  rising  two  01 
three  inches  above  the  jar,  and  terminated  at  the  top  with  a 


291.  What  is  the  appearance  of  electrical  light  through  a  prism?  292.  Describe 
Fig.  269.  and  explain  the  principle  on  which  its  motion  depends.  293.  Suppose  one 
part  or  side  of  an  electric  is  positive,  what  will  be  the  electrical  state  of  the  other  side 
or  parti  What  part  of  the  electrical  apparatus  is  constructed  on  this  principle  1 
How  is  the  Leyden  vial  constructed  ?  Why  is  not  the  whole  surface  of  this  vial  cov- 
ered with  the  tin  foil ) 


348  ELECTRICITY. 

brass  ball,  which  is  called  the  knob  of  the  jar,  is  made  to  de- 
scend through  the  cover,  till  it  touches  the  interior  coating.  It 
is  along  this  rod  that  the  charge  of  electricity  is  conveyed  to 
the  inner  coating,  while  the  outer  coating  is  made  to  communi- 
cate with  the  ground. 

FIG.  270.  FIG.  271. 


Leyden  Jars. 

294.  When  a  chain  is  passed  from  the  prime  conductor  of  an 
electrical  machine  to  this  rod,  the  electricity  is  accumulated  on 
the  tin  foil  coating,  while  the  glass  above  the  tin  foil  prevents 
its  escape,  and  thus  the  jar  becomes  charged.     By  connecting 
togetl1  jr  a  sufficient  number  of  these  jars,  any  quantity  of  the 
electric  fluid  may  be  accumulated.     For  this  purpose,  all  the 
inferior  coatings  of  the  jars  are  made  to  communicate  with  each 
other,  by  metallic  rods  passing  between  them,  and  finally  ter- 
minating in  a  single  rod.     A  similar  union  is  also  established, 
by  connecting  the  external  coats  with  each  other.     When  thus 
arranged,  the  whole  series  may  be  charged,  as  if  they  formed 
but  one  jar,  and  the  whole  series  may  be  discharged  at  the 
same  instant.     Such  a  combination  of  jars  is  termed  an  electri- 
cal battery. 

295.  For  the  purpose  of  making  a  direct  communication  be- 
tween the  inner  and  outer  coating  of  a  single  jar,  or  battery,  by 
which  a  discharge  is  effected,  an  instrument  called  a  discharg- 
ing-rod  is  employed.     It  consists  of  two  bent  metallic  rods, 
terminated  at  one  end  by  brass  balls,  and  at  the  other  end  con- 
nected by  a  joint.     This  joint  is  fixed  to  the  end  of  a  glass 

294.  How  is  a  Leyden  vial  charged  ?  In  what  manner  may  a  number  of  these  vials 
be  charged  ?  What  is  an  electrical  battf  ry  '!  295.  Explain  the  design  of  Fig.  271,  and 
show  how  an  equilibrium  is  produced  by  the  discharging-rod. 


ELECTRfCITY.  349 

handle,  and  the  rods  being  movable  at  the  joint,  the  balls  can 
be  separated  or  brought  near  each  other,  as  occasion  requires. 
When  opened  to  a  proper  distance,  one  ball  is  made  to  touch 
the  tin  foil  on  the  outside  of  the  jar,  and  then  the  other  is 
brought  into  contact  with  the  knob  of  the  jar,  as  seen  in  Fig. 
271.  In  this  manner  a  discharge  is  effected,  or  an  equilibrium 
produced  between  the  positive  and  negative  sides  of  the  jar. 

296.  When  it  is  desired  to  pass  the  charge  through  any  sub- 
stance for  experiment,  then  an  electrical  circuit  must  be  estab- 
lished, of  which  the  substance  to  be  experimented  upon  must 
form  a  part.     That  is,  the  substance  must  be  placed  between 
the  ends  of  two  metallic  conductors,  one  of  which  communicates 
with  the  positive,  and  the  other  with  the  negative  side  of  the 
jar,  or  battery. 

297.  When  a  person  takes  the  electrical  shock  in  the  usual 
manner,  he  merely  takes  hold  of  the  chain  connected  with  the 
outside  coating,  and  the  battery  being  charged,  touches  the 
knob  with  his  finger,  or  with  a  metallic  rod.     On  making  this 
circuit,  the  fluid  passes  through  the  person  from  the  positive  to 
the  negative  side. 

Any  number  of  persons  may  receive  the  electrical  shock,  by 
taking  hold  of  each  other's  hand,  the  first  person  touching  the 
knob,  while  the  last  takes  hold  of  a  chain  connected  with  the 
external  coating.  In  this  manner,  hundreds,  or,  perhaps,  thou- 
sands of  persons,  will  feel  the  shock  at  the  same  instant,  there 
being  no  perceptible  interval  in  the  time  when  the  first  and  the 
last  person  in  the  circle  feels  the  sensation  excited  by  the  passage 
of  the  electric  fluid. 

298.  Atmospheric  Electricity. — The  atmosphere  always  con- 
tains more  or  less  electricity,  which  is  sometimes  positive,  and 
at  others  negative.     It  is,  however,  most  commonly  positive, 
and  always  so  when  the  sky  is  clear  or  free  from  clouds  or  fogs. 
It  is  always  stronger  in  winter  than  in  summer,  and  during  the 
day  than  during  the  night.     It  is  also  stronger  at  some  hours 
of  the  day  than  at  others ;  being  strongest  about  9  o'clock  in 
the  morning,  and  weakest  about  the  middle  of  the  afternoon. 
These  different  electrical  states  are  ascertained  by  means  of  long 


296.  When  it  is  desired  to  pass  the  electrical  fluid  through  any  substance,  where 
must  it  be  placed  in  respect  to  the  two  sides  of  the  battery  ?  297.  Suppose  the  bat- 
tery is  charged,  what  must  a  person  do  to  take  the  shock?  What  circumstance  ii 
related,  which  shows  the  surprising  velocity  with  which  electricity  is  transmitted  1 
293.  Is  the  electricity  of  the  atmosphere  positive  or  negative?  At  what  times  doea 
the  atmosphere  cont'ain  most  electricity?  How  are  the  different  electrical  states  of 
the  atmosphere  ascertained  1 


350  ELECTRICITY. 

metallic  wires  extending  from  one  building  to  another,  and  con- 
nected with  electrometers. 

299.  It  was  .proved  by  Dr.  Franklin,  that  the  electric  fluid 
and  lightning  are  the  same  substance,  and  this  identity  has 
been  confirmed  by  subsequent  writers  on  this  subject. 

If  the  properties  and  phenomena  of  lightning  be  compared 
with  those  of  electricity,  it  will  be  found  that  they  differ  only  in 
respect  to  degree.  Thus,  lightning  passes  in  irregular  lines 
through  the  air ;  the  discharge  of  an  electrical  battery  has  the 
same  appearance.  Lightning  strikes  the  highest  pointed  ob- 
jects— takes  in  its  course  the  best  conductors — sets  fire  to  non- 
conductors, or  rends  them  in  pieces,  and  destroys  animal  life ; 
all  of  which  phenomena  are  caused  by  the  electric  fluid. 

300.  LIGHTNING  RODS. — Buildings  may  be  secured  from  the 
effects  of  lightning,  by  fixing  to  them  a  metallic  rod,  which  is 
elevated  above  any  part  of  the  edifice  and  continued  to  the 
moist  ground,  or  to  the  nearest  water.     Copper,  for  this  pur- 
pose, is  better  than  iron,  not  only  because  it  is  less  liable  to  rust, 
but  because  it  is  a  better  conductor  of  the  electric  fluid.     The 
upper  part  of  the  rod  should  end  in  several  fine  points,  which 
must  be  covered  with  some  metal  not  liable  to  rust,  such  as 
gold,  platina,  or  silver. 

301.  No  protection  is  afforded  by  the  conductor,  unless  it  is 
continued  without  interruption  from  the  top  to  the  bottom  of 
the  building,  and  it  can  not  be  relied  on  as  a  protector,  unless 
it  reaches  the  moist  earth,  or  ends  in  loater  connected  with  the 
earth.     Conductors  of  copper  may  be  three-fourths  of  an  inch 
in  diameter,  but  those  of  iron  should  be  at  least  an  inch  in 
diameter.     In  large   buildings,    complete   protection   requires 
many  lightning  rods,  or  that  they  should  be  elevated  to  a 
height  above  the  building  in  proportion  to  the  smallness  of  their 
numbers,  for  modern  experiments  have  proved  that  a  rod  only 
protects  a  circle  around  it,  the  radius  of  which  is  equal  to  twice 
its  length  above  the  building. 

302.  Thus  a  rod  20  feet  above  the  building,  will  protect  a 
space  of  40  feet  from  it  in  all  directions. 

299.  Who  first  discovered  that  electricity  and  lightning  are  the  same  ?  What  phe- 
nomena are  mentioned  which  belong  in  common  to  electricity  and  lightning?  300. 
How  may  buildings  be  protected  from  the  effects  of  lightning?  Which  is  the  best 
conductor,  iron  or  copper?  301.  What  circumstances  are  necessary,  that  the  rod 
mav  be  relied  on  as  a  protector!  302.  What  diameter  will  a  rod  20  feet  above  th« 
building  protect  ? 


MAGNETISM.  851 

-CHAPTER  XIV. 

MAGNETISM. 

303.  The  native  Magnet,  or  Loadstone,  is  an  ore  of  iron, 
which  is  found  in  various  parts  of  the  world.     Its  color  is  iron 
black ;  its  specific  gravity  from  4  to  5,  and  it  is  sometimes 
found  in  crystals. 

This  substance,  without  any  preparation,  attracts  iron  and 
steel,  and  when  suspended  by  a  string,  will  turn  one  of  its  sides 
toward  the  north,  and  another  toward  the  south. 

It  appears  that  an  examination  of  the  properties  of  this 
species  of  iron  ore,  led  to  the  important  discovery  of  the  mag- 
netic needle,  and  subsequently  laid  the  foundation  for  the 
science  of  magnetism ;  though  at  the  present  day  magnets  are 
made  without  this  article. 

304.  The  whole  science  of  magnetism  is  founded  on  the  fact, 
that  pieces  of  iron  or  steel,  after  being  treated  in  a  certain  man- 
ner, and  then  suspended,  will  constantly  turn  one  of  their  ends 
toward  the  north,  and  consequently  the  other  toward  the  south. 
The  same  property  has  been  more  recently  proved  to  belong  to 
the  metals  nickel  and  cobalt,  though  with  much  less  intensity. 

305.  Still  more  recently,  it  has  been  found  by  Prof.  Faraday, 
that  when  a  strong  electro-magnet  is  employed,  the  following 
metals  are  acted  upon  with  varying  intensity,  and  therefore 
must  be  added  to  the  list  of  magnetic  metals,  viz.,  manganese, 
chromium,  cerium,  titanium,  palladium,  platinum,  and  osmium. 

306.  The  poles  of  a  magnet  are  those  parts  which  possess  the 
greatest  power,  or  in  which  the  magnetic  virtue  seems  to  be 
concentrated.     One  of  the  poles  points  north,  and  the  other 
south.     The  magnetic  meridian  is  a  vertical  circle  in  the  heavens, 
which  intersects  the  horizon  at  the  points  to  which  the  magnetic 
needle,  when  at  rest,  directs  itself. 

307.  The  axis  of  a  magnet,  is  a  right  line  which  passes  from 
one  of  its  poles  to  the  other. 

The  equator  of  a  magnet,  is  a  line  perpendicular  to  its  axis, 
arid  is  at  the  center  between  the  two  poles. 

303.  What  is  the  native  magnet  or  loadstone  ?  What  are  the  properties  of  the 
loadstone  \  304.  On  what  is  tlie  whole  subject  of  magnetism  founded  ?  What  other 
metals  besides  iron  possess  the  magnetic  property?  305.  What  metals  besides  iron, 
nickel,  and  cobalt,  are  magnetic  ?  306.  What  are  the  poles  of  a  magnet  7  307.  What 
is  the  axis  of  a  magnet  1  What  is  the  equator  of  a  magnet  7 


352  MAGNETISM. 

308.  Leading  Properties. — The   leading  properties  of  the 
magnet  are  the  following :    It  attracts  iron  and  steel,  and  when 
suspended  so  as  to  move  freely,  it  arranges  itself  so  as  to  point 
north  and  south  ;  this  is  called  the  polarity  of  the  magnet. 
•When  the  south  pole  of  one  magnet  is  presented  to  the  north 
pole  of  another,  they  will  attract  each  other ;  this  is  called  mag- 
netic attraction.     But  if  the  two  north,  or  two  south  poles  be 
brought  together,  they  will  repel  each  other,  and  this  is  called 
magne  tic  rep  u  Ision. 

309.  When  a  magnet  is  left  to  move  freely,  it  does  not  lie  in 
a  horizontal  direction,  but  one  pole  inclines  downward,  and  con- 
sequently the  other  is  elevated  above  the  line  of  the  horizon. 
This  is  called  the  dipping,  or  inclination  of  the  magnetic  needle. 
Any  magnet  is  capable  of  communicating  its  own  properties  to 
iron  or  steel,  and  this,  again,  will  impart  its  magnetic  virtue  to 
another  piece  of  steel,  and  so  on  indefinitely. 

310.  If  a  piece  of  iron  or  steel  be  brought  near  one  of  the 
poles  of  a  magnet,  they  will  attract  each  other,  and  if  suffered 
to  come  into  contact,  will  adhere  so  as  to  require  force  to  sep- 
arate them.     This  attraction  is  mutual ;  for  the  iron  attracts  the 
magnet  with  the  same  force  that  the  magnet  attracts  the  iron. 
This  may  be  proved,  by  placing  the  iron  and  magnet  on  pieces 
of  wood  floating  on  water,  when  they  will  be  seen  to  approach 
each  other  mutually. 

311.  force  of  Attraction. — The  force  of  magnetic  attraction 
varies  with  the  distance  in  the  same  ratio  as  the  force  of  gravity ; 
the  attracting  force  being  inversely  as  the  square  of  the  distance 
between  the  magnet  and  the  iron. 

312.  The  magnetic  force  is  not  sensibly  affected  by  the  in- 
terposition of  any  substance  except  those  containing  iron,  or 
steel.     Thus,  if  two  magnets,  or  a  magnet  and  piece  of  iron, 
attract  each  other  with  a  certain  force,  this  force  will  be  the 
same  if  a  plate  of  glass,  wood,  or  paper,  be  placed  between 
them.     Neither  will  the  force  be  altered,  by  placing  the  two 
attracting  bodies  under  water,  or  in  the  exhausted  receiver  of 
an  air-pump.     This  proves  that  the  magnetic  influence  passes 
equally  well  through  air,  glass,   wood,  paper,  water,  and   a 
vacuum. 


308.  What  is  meant  by  the  polarity  of  a  magnet  ?  When  do  two  magnets  attract, 
and  when  repel  each  other?  309.  What  is  understood  by  the  dipping  of  the  mag- 
netic needle  1  310.  How  is  it  proved  that  the  iron  attracts  the  magnet  with  the  same 
force  that  the  majrnet  attracts  the  iron  1  31 1.  How  does  the  force  of  magnetic  attrac- 
tion vary  with  the  distance  ?  312.  Does  the  magnetic  force  vary  with  the  interposi- 
tion of  any  substance  between  the  attracting  bodies  J 


MAGNETISM.  353 

313.  Destroyed  by  Heat. — Heat  weakens  the  attractive  power 
of  the  magnet,  and  a  white  heat  entirely  destroys  it.     Electricity 
will  change  the  poles  of  the  magnetic  needle,  and  the  explosion 
of  a  small  quantity  of  gunpowder  on  one  of  the  poles,  will  have 
the  same  effect. 

314.  The  attractive  power  of  the  magnet  may  be  increased 
by  permitting  a  piece  of  steel  to  adhere  to  it,  and  then  suspend- 
ing to  the  steel  a  little  additional  weight  every  day,  for  it  will 
sustain,  to  a  certain  limit,  a  little  more  weight  on  one  day  than 
it  would  on  the  day  before. 

315.  Small  natural  magnets  will  sustain  more  than  large 
ones  in  proportion  to  their  weight.     It  is  rare  to  find  a  natural 
magnet,  weighing  20  or  30  grains,  which  will  lift  more  than 
thirty  or  forty  times  its  own  weight.     But  a  minute  piece  of 
natural  magnet,  worn  by  Sir  Isaac  Newton,  in  a  ring,  which 
weighed  only  three  grains,  is  said  to  have  been  capable  of  lifting 
746  grains,  or  nearly  250  times  its  own  weight. 

316.  Artificial  Magnets. — The  magnetic  property  may  be 
communicated  from  the  loadstone,  or  artificial  magnet,  in  the 
following  manner,  it  being  understood  that  the  north  pole  of 
one  of  the  magnets  employed,  must  always  be  drawn  toward  the 
south  pole  of  the  new  magnet,  and  that  the  south  pole  of  the 
other  magnet  employed,  is  to  be  drawn  in  the  contrary  direc- 
tion.    The  north  poles  of  magnetic  bars  are  usually  marked 
with  a  line  across  them,  so  as  to  distinguish  this  end  from  the 
other. 

Place  two  magnetic  bars 
A  and  B,  Fig.  272,  so  that 
the  north  end  of  one  may 
be  nearest  the  south  end 
of  the  other,  and  at  such 
a  distance  that  the  ends  of  _,______»_ 

the  steel  bar  to  be  touched,        \MwJ  c 

may  rest  upon  them.  Hav-  Artificial  Magnets. 

ing  thus  arranged  them, 
as  shown  in  the  figure,  take  the  two  magnetic  bars,  D  and  E, 
and  apply  the  south  end  of  E,  and  the  north  end  of  D,  to  the 
middle  of  the  bar  C,  elevating  their  ends  as  seen  in  the  figure. 
Next  separate  the  bars  E  and  D,  by  drawing  them  in  opposite 
directions  along  the  surface  of  C,  still  preserving  the  elevation 

313.  What  is  the  effect  of  heat  on  the  magnet  1  What  is  the  effect  of  electricity,  or 
the  explosion  of  gunpowder  on  it  1  314.  How  may  the  power  of  a  magnet  be  in- 
creased ?  315.  What  is  said  concerning  the  comparative  powers  of  great  and  small 
magnets  1  316.  Explain  Fig.  272,  and  describe  the  mode  of  making  magnet. 


354  MAGNETISM. 

of  their  ends  ;  then  removing  the  bars  D  and  E,  to  the  distance 
of  a  foot  or  more  from  the  bar  C,  bring  their  north  and  south 
poles  into  contact,  and  then  having  again  placed  them  on  the 
middle,  C,  draw  them  in  contrary  directions,  as  before.  The 
same  process  must  be  repeated  many  times  on  each  side  of  the 
bar,  C,  when  it  will  be  found  to  have  acquired  a  strong  and  per- 
manent magnetism. 

317.  If  a  bar  of  iron  be  placed,  for  a  long  period  of  time, 
in  a  north  and  south  direction,  or  in  a  perpendicular  position, 
it  will  often  acquire  a  strong  magnetic  power.     Old  tongs, 
pokers,  and  fire  shovels,  almost  always  possess  more  or  less 
magnetic  virtue  ;  and  the  same  is  found  to  be  the  case  with  the 
iron  window  bars  of  ancient  houses,  whenever  they  have  hap- 
pened to  be  placed  in  the  direction  of  the  magnetic  line. 

318.  A  magnetic  needle,  such  as  is  employed  in  the  mariner's 
and  surveyor's  compass,  may  be  made  by  fixing  a  piece  of  steel 
on  a  board,  and  then  drawing   two   magnets  from  the  center 
toward  each  end,  as  directed  at  Fig.  272.     Some  magnetic 
needles,  in  time,  lose  their  virtue,  and  require  again  to  be  mag- 
netized.    This  may  be  done  by  placing  the  needle  still  suspend- 
ed on  its  pivot,  between  the  opposite  poles  of  two  magnetic 
bars.     While  it  is  receiving  the  magnetism,  it  will  be  agitated, 
moving  backward  and  forward,  as  though  it  were  animated  ; 
but  when  it  has  become  perfectly  magnetized,  it  will  remain 
quiescent. 

FIG.  273. 


Magnetic  Rotation. 

319.  Magnetic  Rotation.  —  It  is  quite  interesting  to  observe 
the  different"  directions  the  needle  of  a  small  magnetic  compass 

317.  In    what    positions    do    bars    of  iron    become    magnetic   spontaneously? 
318.  How  may  a  needle  he  magneti/.ed  without  removing  from  its  pivot  1 


MAGNETISM. 


355 


will  assume  when  moved  round  a  bar  magnet.  If  the  latter  be 
laid  on  the  table,  and  the  former  carried  slowly  around  it,  from 
S,  or  south,  to  N,  or  north,  and  so  back  again  on  the  other 
side,  the  needle  will  alternately  take  all  the  positions  shown 
by  Fig.  273. 

320.  DIP  OF  THE  MAGNET. — The  dip,  or  inclination  of  the 
magnetic  needle,  is  its  deviation  from  its  horizontal  position,  as 
already  mentioned.     A  piece  of  steel,  or  a  needle  which  will 
rest  on  its  center,  in  a  direction  parallel  to  the  horizon,  before 
it  is  magnetized,  will  afterward  incline  one  of  its  ends  toward 
the  earth.     This  property  of  the  magnetic  needle  was  discov- 
ered by  a  compass-maker,  who,  having  finished   his  needles 
before  they  were  magnetized,  found  that  immediately  after- 
ward, their  north  ends  inclined  toward  the  earth,  so  that  he 
was  obliged -to  add  small  weights  to  their  south  poles,  in  order 
to  make  them  balance,  as  before. 

321.  The  dip  of  the  magnetic  needle  is  measured,  by  a  grad- 
uated circle,  placed  in  the  vertical  position,  with  the  needle 
suspended  by  its  side.     Its  inclination  from  a  horizontal  line, 
marked  across  the  face  of  this  circle,  is  the 

measure  of  its  dip.     The  circle,  as  usual,  FIG.  274. 

is  divided  into  360  degrees,  and  these  into 
minutes  and  seconds.  « 

322.  Dipping  Needle. — Fig.  274  is  said 
to  represent  a  convenient  form  of  the  dip- 
ping needle.     It  is  a  strongly  magnetized 
steel  needle,  turning  on  the  center  of  grav- 
ity A  B,  in  a  brass  frame  which  is  suspend- 
ed  by  a  thread.     Thus   the   needle   has 
universal  motion.     The  scale  is  omitted  as 
unnecessary  for  the  present  purpose. 

323.  The   dip   of  the   needle  does  not 
vary  materially  at  the  same  place,  but  dif- 
fers in  different  latitudes,  increasing  as  it  is 
carried  toward  the  north,  and  diminishing 
as   it  is   carried   toward  the   south.      At 
London,  the  dip  for  many  years  has  varied 
little  from  72  degrees.     In  the  latitude  of 
80  degrees  north,  the  dip,  according  to  the 
observations  of  Captain  Parry  was  88  de-         Dipping  Needle. 
grees. 

320.  How  was  the  dip  of  the  magnetic  needle  first  discovered?    321.  In  what 
manner  is  the  dip  measured  7 


356 


MAGNETISM. 


324.  VARIATION   OF  THE    MAGNET. — Although,  in  general 
terms,  the  magnetic  needle  is  said  to  point  north  and  south,  yet 
this  is  very  seldom  strictly  true,  there  being  a  variation  in  its 
direction,  which  differs  in  degree  at  different  times  and  places. 
This  is  called  the  variation,  or  declination,  of  the  magnetic 
needle. 

325.  This  variation  is  determined  at  sea,  by  observing  the 
different  points  of  the  compass  at  which  the  sun  rises,  or  sets, 
and  comparing  them  with  the  true  points  of  the  sun's  rising  or 
setting,  according  to  astronomical  tables.     By  such  observa- 
tions it  has  been  ascertained  that  the  magnetic  needle  is  contin- 
ually declining  alternately  to  the  east  or  west  from  due  north, 
and  that  this  variation  differs  in  different  parts  of  the  world  at 
the  same  time  and  at  the  same  place  at  different  times. 

326.  The  annexed  table  shows  at  once,  the  dip,  or  inclina- 
tion, and  the  variation  or  declination  of  the  needle,  for  a  series 
of  years.     It  was  formed  from  observations  made  at  Brussels, 
and  by  it  there  appears  to  be  a  gradual,  but  constant  diminu- 
tion of  the  angle,  both  of  inclination  and  declination,  in  Europe. 


Month. 

Year. 

Inclination. 

Declination. 

October,  .     . 

1827, 

*    680,  56',  5" 

220,  28',  8" 

March,  .     ... 

1830, 

680,  52',  6" 

220,  25',  3" 

March,     .     . 

1832, 

680,  49/,  i// 

220,  19/,  0" 

March,       .     . 

1833, 

680,  42/,  8" 

220,  is/,  4// 

April,       .     . 

1834, 

680,  38/,  4" 

22°,  15',  2" 

March,  .     .     . 

1835, 

680,  ss/  0" 

220,    6',  7" 

March,     .     . 

1836, 

680,  32/,  2" 

220,    7/;6" 

March,  .     .     . 

1837, 

680,  28',  8" 

220,    4/,  3// 

March,     .     . 

1838, 

680,  26',  1" 

220,    3/,7/x 

March,  .     .     . 

1839, 

680,  22',  4" 

210,  53/  6'' 

327.  The  difference  in  the  declination,  which  may  be  of 
much  importance,  as  on  it  may  depend  the  safety  of  ships  at 
sea,  is  very  material  in  different  countries,  and  at  different 
periods.  Thus  at  present  it  is  about  24°  west,  at  London. 
At  Paris,  22°  west.  At  New  York,  5°  25'  west,  and  at  Hart- 
ford, about  6°  west. 

Before  1660,  the  variation  at  London,  was  toward  the  east, 
and  on  that  year  the  needle  pointed  due  north.  From  that 

323.  What  circumstance  increases  or  diminishes  the  dip  of  the  needle  7  324.  What 
is  meant  hy  the  declination  of  the  magnetic  needle?  326.  What  changes  does  the 
above  table  indicate  7  327.  Why  is  the  difference  of  declination  of  importance  to 
ships  1  i 


ELECTRO-MA  GXETISM. 


357 


time  to  the  present,  it  has  gained  from  two  to  six  degrees  to- 
ward the  west  every  year. 

The  greatest  variation  of  the  magnetic  needle,  recorded,  was 
that  observed  by  Capt.  Cook,  which  was  about  43°  west.  This 
was  in  S.  lat.  60°,  and  E.  longitude,  92°  36'. 


CHAPTER  XV. 

ELECTRO. MAGNETISM. 

328.  When  two  metals,  one  of  which  is  more  easily  oxyda- 
ted  than  the  other,  are  placed  in  acidulated  water,  and  the  two 
metals  are  made  to  touch  each  other,  or  a  metallic  communica- 
tion is  made  between  them,  there  is  excited  an  electrical  or  gal- 
vanic current,  which  passes  from  the  metal  most  easily  oxy dated, 
through  the  water,  to  the  other  metal,  and  from  the  other  metal 
through  the  water  around  to  the  first  metal  again,  and  so  in  a 
perpetual  circuit. 


FIG.  275. 


FIG.  276. 


Galvanic  Current. 


Galvanic  Battery. 


o2vJ.  If  we  take,  for  example,  one  slip  of  zinc,  and  another 
of  copper,  and  place  them  in  a  cup  of  diluted  sulphuric  acid, 
Fig.  275,  tiieir  upper  ends  in  contact  and  above  the  water,  and 


328.  What  conditions  aie  necessary  to  excite  the  galvanic  action  ?    From  which 
metal  does  the  galvanism  proceed  7    329.  Describe  the  circuit  by  Fig.  275. 


358 


ELECTRO-MAGNETISM. 


their  lower  ends  separated,  then  there  will  be  constituted  a 
galvanic  circle,  of  the  simplest  form,  consisting  of  three  ele- 
ments, zinc,  acid,  copper.  The  galvanic  influence  being  excited 
by  the  acid,  will  pass  from  the  zinc  Z,  the  metal  most  easily 
oxydated,  through  the  acid,  to  the  copper  C,  and  from  the  cop- 
per to  the  zinc  again,  and  so  on  continually,  until  one  or  the 
other  of  the  elements  is  destroyed,  or  ceases  to  act. 

The  same  effect  \vill  be  produced,  if  instead  of  allowing  the 
metallic  plates  to  come  in  contact,  a  communication  between 
them  be  made  by  means  of  wires,  as  shown  by  Fig.  276.  In 
this  case,  as  well  as  in  the  former,  the  electricity  proceeds  from 
the  zinc  Z,  which  is  the  positive  side,  to  the  copper  C,  being 
conducted  by  the  wires  in  the  direction  shown  by  the  arrows. 

330.  The  completion  of  the  circuit  by  means  of  wires  enables 
us  to  make  experiments  on  different  substances  by  passing  the 
galvanic  influence  through  them,  this  being  the  method  em- 
ployed to  exhibit  the  effects  of  galvanic  batteries,  and  by  which 
the  most  intense  heat  may  be  produced. 

When  the  two  poles  of  a  battery  are  connected  by  means  of 
a  copper  wire  of  a  yard  or  two  in  length,  the  two  parts  being 
supported  on  a  table  in  a  north  and  south  direction,  for  some 
of  the  experiments,  but  in  others  the  'direction  must  be  changed 
as  will  be  seen.  This  wire,  it  will  be  remembered,  is  called  the 
uniting  wire. 

331.  Theory. — In  theory,  the 

r'tive  electricity  is  produced 
the  mutual  action  of  the 
acid,  water,  and  zinc ;  the  water, 
in  small  quantity,  being  decom- 
posed. If  this  action  is  too  vio- 
lent, that  is,  if  the  acid  is  too 
strong  and  the  hydrogen  pro- 
duced in  too  large  quantity,  the 
electrical  current  is  diminished, 
or  ceases  almost  entirely. 

332.  GALVANIC    BATTERY. — 
One    of    the    most    convenient 
forms  of  a  galvanic  battery  for  ex- 
periments described  in  this  work 
is  represented  by  Fig.  277.      It 
consists   of  a  cylinder  of  sheet 
copper,  within  which  is  another 

£U.  How  is  positive  dectrieity  produced  1 


FIG.  277. 


ELECTRO-MAGNETISM.  359 

of  zinc.  The  zinc  has  for  its  bottom  a  piece  of  sheep-skin,  or 
bladder,  tied  on  with  a  string,  and  is  suspended  an  inch  or  two 
from  the  bottom  of  the  copper  cylinder.  Or,  the  whole  inner 
cylinder  may  be  made  of  leather  with  a  slip  of  zinc  within  it. 
This  is  done  to  prevent  the  fluid  which  the  inner  cylinder  contains 
from  mixing  with  that  contained  between  the  two ;  and  still,  the 
leather  being  porous,  the  water  it  contains  conducts  the  galvanic 
influence  from  one  cell  to  the  other,  as  already  stated.  The 
diameter  of  the  outer  cup  may  be  five  or  six  inches,  and  the 
inner  one  three  or  four.  The  zinc  may  be  suspended  by  making 
two  holes  near  the  top  and  tying  on  a  piece  of  glass  tube  or  a 
slip  of  wood.  This  part  has  often  to  be  removed  and  cleaned, 
by  scraping  off  the  black  oxyd,  which,  if  it  remains,  will  pre- 
vent the  action  of  the  battery.  The  action  will  be  sustained 
much  longer  if  the  zinc  is  amalgamated  by  spreading  on  it  a 
little  mercury  before  it  is  used,  and  while  the  surface  is  bright. 

The  cups  P  N",  are  the  positive  and  negative  poles.  They  may 
be  made  of  percussion  caps,  soldered  to  the  ends  of  two  copper 
wires ;  the  other  ends  being  connected  by  soldering,  or  other- 
wise, one  with  the  zinc,  and  the  other  with  the  copper,  cylinder. 

The  inner  cup  is  to  be  filled  with  water,  mixed  with  about  a 
twentieth  part  of  sulphuric  acid,  while  the  cell  between  the  two 
contains  a  saturated  solution  of  sulphate  of  copper,  or  blue 
vitriol.  In  order  to  keep  the  solution  saturated,  especially  when 
casts  are  to  be  taken,  some  of  the  solid  vitriol  is  to  be  tied  in  a 
rag  and  suspended  in  it. 

This  battery,  it  will  be  seen,  differs  materially  from  that 
hereafter  to  be  described  under  the  name  of  Grove's  battery, 
but  for  common  purposes  it  is  equally  useful ;  is  much  more 
readily  made,  and  costs  only  a  tenth  as  much. 


GROVE'S    BATTERY. 


333.  This  is  the  most  powerful  arrangement,  according  to  its 
size  and  cost,  which  has  been  proposed,  and  is  that  generally  used 
for  telegraphic  purposes.  Fig.  278  shows  a  battery  of  twelve 
cups,  each  of  which  consists  of  a  cylinder  of  amalgamated 
zinc,  within  which  is  a  cup  of  unglazed  clay  ;  these  being  placed 
within  an  outer  cup  of  glass.  To  the  zinc  is  attached  a  con- 
ducting arm  of  the  same  metal,  which  reaches  to  the  next 
series  of  cups,  and  at  the  end  of  which  is  attached  a  thin  piece  of 
platina,  which  dips  into  the  porous  cup,  as  shown  by  the  figure. 

332.  Explain  Fisr.  277,  and  show  the  action  ef  the  battery.    333.  Describe  the  prin- 
ciple of  Grove's  battery,  Fig,  278, 


360 


ELECTRO-MAGNETISM. 
FIG.  278. 


&rave'a  Twevle-Cup  Battery. 

The  battery  is  charged,,  by  filling  the  clay  cup  with  nitric 
acid,  and  the  space  within  and  around  the  zinc,  which  is  open 
at  the  bottom  and  side,  with  sulphuric  acid,  diluted  with  30 
parts  of  water.  The  action  is  strong,  and  requires  very  little 
expense. 

334.  Tobacco  Pipe  Battery. — For  telegraphic  batteries  the 
vessels  are  about  four  inches  high,  but  for  common  experiments 
any  one  may  make  a  miniature  battery  in  the  following  man- 
ner, and  at  a  very  trifling  expense. 

Procure  six  toy  tumblers,  an  inch  and   a  half  high.     Cut 
from    sheet  zinc,  strips   of    such 
size     as    to   form     cylinders     to  FIG.  279. 

go  within  these  tumblers.  Cut 
one  end  of  each  strip  nearly  off, 
and  a  quarter  of  an  inch  wide, 
as  shown  at  A,  Fig.  279,  and 
turn  it  up  so  as  to  make  a  con- 
necting arm  with  the  next  cup. 
At  the  end  of  this  arm  cut  a  slit 
B,  into  which  put  a  little  slip  of 
platina  foil,  half  an  inch  wide,  and 
an  inch  long.  In  this  manner  the 
whole  can  be  made  without  sold- 
ering the  arm  to  the  CUp,  which,  Tobacco  Pipe  Battery. 
when  amalgamated,  will  drop  off. 

Next  take  six  tobacco  pipes,  and  breaking  off  the  stems,  stop 
the  orifices  of  the  bowls  with  sealing-wax,  and  the  elements  of 
your  little  battery  is  finished. 

Now  take  a  little  mercury  in  a  bowl,  and  touching  the  zinc 


334.  How  is  the  tobacco  pipe  battery  made? 


ELECTRO-MAGNETISM. 


361 


cylinders  to  it,  a  little  will  adhere  to  the  metal,  and  may  be 
spread  over  its  surface  with  a  wisp  of  cotton.  The  action  is 
thus  much  increased. 

335.  Lastly,  put  the  bowls  within  the  zinc  cylinders,  and  these 
into  the  tumblers,  and  then  fill  the  bowls  with  nitric  acid,  and 
the  tumblers  with  sulphuric  acid  diluted  with  30  parts  of  water, 
fixing  the  arms  so  that  the  platina  will  dip  into  the  bowls,  and 
the  action  will  commence  instantly. 

AVith  this  little  battery,  which  any  one  of  ordinary  ingenuity 
can  make,  all  the  common  experiment  with  a  galvanic  battery 
may  be  performed. 

336.  Circular  Motion    of  Electro- Magnetism. — In   conse- 
quence   of  the    circular    magnetic    currents   which    seem    to 
emanate  from  the  regular  influence 

of   the   battery,  the    fluid   may  be  FIG.  280. 

made  to  act  so  as  to  produce  a 
continued  rotation  of  the  conducting 
wire,  or  the  magnet. 

Magnet  Revolving  Around  the 
Conducting  Wire. — The  arrangement 
shown  by  Fig.  280,  and  which  causes 
the  magnet  to  revolve  around  the 
conducting  wire,  consists  of  the  mag- 
net N  S,  having  an  angular  bend  in 
the  middle,  where  it  becomes  hori- 
zontal, while  the  extremities  are  vert- 
ical. To  the  north  pole,  or  lower 
end  of  the  magnet  N,  is  attached  a 
piece  of  brass,  at  a  right-angle  with 
the  magnet,  which  has  a  little  pro- 
jection, forming  a  pivot,  which  rests 
in  an  agate  cup,  fixed  to  the  stand. 
A  wire  loop  attached  to  the  upper 
pole  of  the  magnet  S,  encircles  the 
conducting  wire,  and  thus  keeps  the 
magnet  in  its  place.  The  galvanic 
current  is  conveyed  by  this  wire,  the 
lower  end  of  which  dips  into  a  little 
cup  of  mercury  on  the  horizontal  Revolving  Magnet. 

portion  of  the  magnet. 

337.  The  wire  has  a  brass  cup  at  A,  containing  mercury,  and 
into  which  the  pole  of  the  battery  is  inserted.     From  this  cup 


335.  With  what  is  this  battery  charged  ? 

16 


362 


ELECTRO-MAGNETISM. 


FIG.  281. 


projects  a  bent  wire,  as  seen  in  the  figure,  the  end  of  which 
dips  into  a  circular  cistern  of  mercury,  contained  in  a  brass 
cup,  and  through  which  the  magnet  revolves.  A  wire  passes 
through  the  side  of  the  cistern  to  the  mercury,  and  terminates 
in  the  screw-cup  B,  into  which  the  other  pole  of  the  battery  is 
placed. 

Now  on  making  the  connection,  the  current  flows  down  by 
the  side  of  the  upper  pole  of  the  magnet  to  the  middle,  and 
then  takes  the  direction  of  the  cup  B,  so  as  not  to  act  on  the 
lower  pole,  the  galvanic  force  being  between  the  mercury  in 
the  cistern  and  the  bent  wire,  and  by  the  attraction  of  which 
the  magnet  revolves  rapidly  around  the  conducting  wire.  On 
changing  the  poles  the  rotation  will  be  in  a  contrary  direction. 

338.  REVOLVING  SPUR-WHEEL. — Many  curious  experiments 
are  made  by  combining  the  action  of  electricity  with  that  of 
magnetism.     Such  a  combination  is  shown  by  Fig.  281  where 
W  is  a  copper  wheel  cut  into  points,  and   made  to  revolve  be- 
tween the  legs  of  a  U  magnet  fixed  in  an  upright  position. 
The  axis  of  the  wheel  is  supported  by  strips  of  brass  fastened 
to  the  magnetic  poles  N  and  S. 

The  trough  T  may  be  of  brass  or 
wood,  and  is  placed  between  the 
bifurcation  of  the  magnet.  This 
contains  a  little  mercury,  into 
which  the  teeth  of  the  wheel  just 
dip,  as  they  revolve. 

339.  On  the  platform  or  stand, 
to  which  the  magnet  is  fastened, 
are  two  screw-cups  to  which  the 
opposite  poles  of  the  battery  are 
fastened.     One  of  these  cups  is 
connected  with  the  magnet,  and 
through  that,  with  the  axis  of  the 
wheel,   and  the   other  with  the 
mercury   in  the   trough.     Now 
on   making   the  connection   be- 
tween the  poles  of  the  battery, 
the  wheel  begins  to  move,  in  con- 
sequence of    the   attraction   be- 
tween  the  points  of  the  wheel 

and  the  mercury,  and  if  the  cur-  Revolving  wheel. 

rent  is   strong   the  wheel   turns 

with  great  velocity,  snapping  and  striking  fire  as  the  point* 


VIBRATING    WIRE. 


363 


approach  the  fluid  metal.     The  points  of  the  wheel  should  be 
amalgamated  to  make  the  experiment  succeed  well. 


CLOCK-WORK   VIBRATING    WIRE. 


340.  This  is  a  curious  and  singular  arrangement,  and  will 
quite  astonish  those  who  are  not  conversant  with  motions  com- 
municated by  galvanic  influence. 

The  cut,  Pig.  282,  shows  a  connection  between  the  spiral 
ribbon,  A,  and  the  single  Grove's  battery,  B,  by  means  of  a 


FIG.  282. 


Clock-work  Vibrating  Wire. 

copper  wire.  The  bent  wire  C  C,  suspended  in  the  middle,  is 
set  in  motion  by  a  spring  below  the  milled-head  F,  and  is  made 
to  vibrate  rapidly  by  clock-work,  the  ends  of  the  wire  dipping 
alternately  in  the  glass  cups  C  C,  containing  mercury.  The 
spring  is  wound  up  by  turning  the  milled-head. 

The  glass  cups  are  open  at  the  bottom  to  allow  the  mercury 
to  come  in  contact  with  the  brass  pillars  on  which  they  stand. 

Both  of  these  pillars  are  connected  with  one  of  the  screw -cups 
D  D,  while  the  other  cup  is  connected  with  the  middle  brass 
pillar  E,  on  which  is  a  brass  cup  of  mercury.  From  the  latter 
cup  ascends  a  vertical  wire,  attached  to  the  vibrating  wire,  as 
the  figure  shows. 

341.  Such  a  quantity  of  mercury  is  put  into  the  brass  cup 
as  to  keep  the  end  of  the  vertical  wire  covered,  and  enough 
into  the  glass  cups  C  C,  to  allow  one  end  of  the  vibrating  wire 
to  leave  the  mercury  in  the  cup,  before  the  other  end  dips  into 
that  metal. 

342.  The  spiral  ribbon  is  made  by  cutting  strips  of  sheet 
copper,  an  inch  wide,  into  lengths,  and  soldering  them  together. 


864  BELL   ENGINE. 

Then  having  covered  the  whole  with  cotton  cloth,  and  rolled  it 
into  a  spiral,  like  a  watch-spring,  the  article  in  question  is  formed. 
At  each  end,  the  ribbon  being  sometimes  100  feet  long,  there  is 
fixed  a  screw-cup  to  contain  mercury  for  the  poles  of  the  bat- 
tery. In  the  above,  one  end  is  connected  with  the  battery,  and 
the  other  with  the  screw-cup  D,  and  so  to  E,  on  the  platform. 

The  current  must  be  transmitted  through  the  two  instru- 
ments in  succession,  by  connecting  one  of  the  screw-cups  with 
one  of  those  attached  to  the  spiral  wire,  and  the  other  with 
the  pole  of  the  battery  ;  the  remaining  cup  on  the  spiral  being 
made  to  communicate  with  the  other  pole  of  the  battery. 

343.  Action. — On  making  the  connection  with  the  spiral, 
as  shown,  and  turning  the  milled-head  to  put  the  vibrating 
wire  in  motion,  a  brilliant  spark  will  be  seen,  and  a  loud  snap 
heard,  at  the  alternate  rupture  of  the  contact  between  the  ends 
of  the  wire  and  the  mercury  in  the  cups  C  C. 

With  a  battery  of  a  few  pairs  of  large  sized  plates,  the  size 
of  the  spark  will  be  greatly  increased. 

A  strong  shock  may  also  be  given,  especially  when  the  mer- 
cury in  the  cups  C  C  are  covered  with  a  little  oil. 

[The  author  is  indebted  for  the  above,  as  well  as  for  several 
other  cuts  of  the  same  kind,  to  Davis's  "Manual  of  Magnetism," 
Boston,  1850. 

This  work  contains  the  most  complete  and  extensive  set  of 
figures,  and  their  descriptions,  on  the  subjects  of  Magnetism 
and  Electricity,  ever  published  in  this  country.  Price  $1,00. 
The  number  of  figures,  184.] 


REVOLVING    BELL   ENGINE. 


S44.  This  curious  arrangement  is  the  invention  of  Mr.  Page. 
It  consists  of  a  U  shaped  magnet,  the  north  and  south  poles, 
N  S,  being  fixed  in  the  base  board.  Between  these  is  a  small 
electro-magnet  of  iron,  surrounded  with  insulated  copper  wire. 
This  is  fixed  to  a  revolving  axis,  or  wire,  the  upper  end  of 
which  is  confined  in  the  bend  of  the  large  magnet,  and  the 
lower  one  running  in  a  support  below  the  electro-magnet. 
On  the  outside  of  the  U  magnet  are  the  connecting  screws  for 
the  opposite  poles  of  the  battery,  by  which  the  machine  is  oper- 
ated. On  the  axis,  and  connected  with  the  notches  of  the 
wheel,  is  an  endless  screw,  and  with  this  is  connected  the  ham- 
mer, which  strikes  the  bell,  seen  as  a  crown  on  the  figure. 

345.  Action. — The  operation,  or  motion,  of  this  curious  little 


BELL    ENGINE. 


365 


engine,  depends  on  the  alternate  FIG.  283. 

attraction  and  repulson  of  the  poles 
of  the  U  magnet,  and  those  of  the 
small  electro-magnet  bet  ween  them. 
The  magnetism  of  the  latter  de- 
pends on  the  influence  of  the  bat- 
tery with  which  it  is  connected, 
and  therefore  ceases  when  this 
connection  is  broken.  The  revo- 
lution is  therefore  caused  by  the 
mutual  repulsion,  and  then  the 
mutual  attraction  between  the  two 
opposite  poles  of  the  two  magnets, 
as  the  connection  is  broken  and  the 
poles  of  the  electro-magnet  are 
reversed. 

The  hammer  is  made  to  strike 
by  a  pin  on  the  wheel,  moved  by 
the  endless  screw,  and  which  press- 
es back  the  handle  until  it  is  re- 
leased, when  a  spiral  spring  on  the 
handle  impels  it  against  the  bell. 

346.  If  the  wheel  has  100  teeth, 
as  in  the  cut,  the  electro-magnet 
must  revolre  100  times  in  order  to 

produce  one  revolution  of  the  wheel,  and  consequently  one 
stroke  on  the  bell.  The  velocity  of  the  electro -magnet  in  this 
machine,  as  shown  by  the  striking  of  the  hammer,  is  some- 
times equal  to  6000  revolutions  in  a  minute. 

347.  VIBRATION  OF  A  WIRE. — A  conducting  copper  wire 
W,  Fig.  284,  is  suspended  by  a  loop  from  a  hook  of  the  same 
metal,  which  passes  through  the  arm  of  metal  or  wood,  as  seen 
in  the  cut.     The  upper  end  of  the  hook  terminates  in  the  cup 
P  to  contain  mercury.     The  lower  end  of  the  copper  wire  just 
touches  the  mercury,  Q,  contained  in  a  little  trough  about  an 
inch  long,  formed  in  the  wood  on  which  the  horseshoe  magnet, 
M,  is  laid,  the  mercury  being  equally  distant  from  the  two  poles. 

The  cup,  N,  has  a  stem  of  wire  which  passes  through  the 
wood  of  the  platform  into  the  mercury,  this  end  of  the  wire 
being  tinned,  or  amalgamated,  so  as  to  form  a  perfect  contact. 


Bell  Engine. 


347.  Explain  Fig.  2S4,  and  describe  the  course  of  the  electric  fluid  from  one  cup  to 
the  other.    How  must  the  points  of  the  vibrating  wire  be  adjusted  in  order  to  act  7 


366 


BELL    ENGINE. 


Vibration  of  a  Wire. 


348.  Having  thus    prepared 
the  apparatus,  put  a  little  mer- 
cury into  the  cups  P  and  N,  and 
then  form  the  galvanic  circuit 
by  placing  the  poles  of  the  bat- 
tery in  the  two  cups,  and  if  every 
thing  is    as   it   should  be,  the 
wire  will  begin  to  vibrate,  being 
thrown  with  considerable  force 
either  toward  M  or  Q,  accord- 
ing to  the  position  of  the  mag- 
netic poles,  or  the  direction  of 
the  current,  as  already  explained. 
In  either  case  it  is  thrown  out 
of  the  mercury,  and  the  galvanic 
circuit  being  thus  broken,   the 
effect  ceases  until  the  wire  falls 
back  again  by  its  own  weight, 
and  touches  the  mercury,  when 
the  current  being  again  perfect- 
ed, the  same  influence  is  repeated,  and  the  wire  is  again  thrown 
away  from  the  mercury,  and  thus  the  vibratory  motion  becomes 
constant. 

This  forms  an  easy  and  beautiful  electro-magnetic  experi- 
ment, and  may  be  made  by  any  one  of  common  ingenuity, 
who  possesses  a  galvanic  battery,  even  of  small  power,  and  a 
good  magnet. 

The  platform  may  be  nothing  more  than  a  piece  of  pine 
board,  eight  inches  long  and  six  wide,  with  two  sticks  of  the 
same  wood,  forming  a  standard  and  arm  for  suspending  the 
vibrating  wire.  The  cups  may  be  made  of  percussion  caps, 
exploded,  and  soldered  to  the  ends  of  pieces  of  copper  bell 
wire. 

The  wire  must  be  nicely  adjusted  with  respect  to  the  mer- 
cury, for  if  it  strikes  too  deep  or  is  too  far  from  the  surface,  no 
vibrations  will  take  place.  It  ought  to  come  so  near  the  mer- 
cury as  to  produce  a  spark  of  electrical  fire,  as  it  passes  the 
surface,  at  every  vibration,  in  which  case  it  may  be  known  that 
the  whole  apparatus  is  well  arranged.  The  vibrating  wire  must 
be  pointed  and  amalgamated,  and  may  be  of  any  length,  from 
a  few  inches  to  a  foot  or  two. 

349.  ROTATION  OF  A  WHEEL,  similar  to,  but  more  simple,  than 
Fig.  281.     The  same  force  which  throws  the  wire  away  from 


BELL    ENGINE. 


367 


Rotation  of  a  Whed. 


the  mercury,  will  cause  the  ro-  FIG.  255. 

tation  of  a  spur-wheel.  For  this 
purpose  the  conducting  wire,  in- 
stead of  being  suspended,  as  in 
the  former  experiment,  must  be 
fixed  firmly  to  the  arm,  as  shown 
by  Fig.  285.  A  support  for  the 
axis  of  the  wheel  may  be  made 
by  soldering  a  short  piece  to  the 
side  of  the  conducting  wire,  so  as 
to  make  the  form  of  a  fork,  the 
lower  end  of  which  must  be  flat- 
tened with  a  hammer,  and  pierced 
with  fine  orifices,  to  receive  the 
ends  of  the  axis. 

The  apparatus  for  a  revolving- 
wheel  is,  in  every  respect,  like 
that  already  described  for  the  vi- 
brating wire,  except  in  that  above 
noticed,  the  wheel  may  be  made 

of  brass  or  copper,  but  must  be  thin  and  light,  and  so  suspended 
as  to  move  freely  and  easily  The  points  of  the  notches  must 
be  amalgamated,  which  is  done  in  a  few  minutes,  by  placing 
the  wheel  on  a  flat  surface,  and  rubbing  them  with  mercury 
by  means  of  a  cork.  A  little  diluted  acid  from  the  gal- 
vanic batteiy  will  facilitate  the  process.  The  wheel  may  be 
from  half  an  inch  to  several  inches  in  diameter.  A  cent  ham- 
mered thin,  which  may  be  done  by  heating  it  two  or  three 
times  during  the  process,  and  then  made  perfectly  round,  and 
its  diameter  cut  into  notches  with  a  file,  will  answer  every 
purpose. 

This  affords  a  striking  and  novel  experiment;  for  when  every 
thing  is  properly  adjusted,  the  wheel  instantly  begins  to  revolve 
on  touching  with  one  of  the  wires  of  the  battery  the  mercury 
in  the  cup  P,  the  other  pole  being  in  N. 

When  the  poles  of  the  magnet,  or  those  of  the  battery,  are 
changed,  the  wheel  instantly  revolves  in  a  contrary  direction 
from  what  it  did  before. 

It  is.  however,  not  absolutely  necessary  to  divide  the  wheel 
into  notches,  or  rays,  in  order  to  make  it  revolve,  though  the 


349.  Explain  Fig.  285.  In  what  manner  may  the  points  of  the  spur-wheel  be  amal- 
gamated 1  If  the  motion  of  the  fluid  is  changed,  what  effect  does  it  have  on  the 
wheel  7 


368  BELL   ENGINE. 

motion  is  more  rapid,  and  the  experiment  succeeds  much  better 
by  doing  so. 

350.  ELECTRO-MAGNETIC    INDUCTION. — Experiment  proves 
that  the  passage  of  the  galvanic  current  through  a  copper  wire 
renders  iron  magnetic  when  in    the   vicinity  of  the  current. 
This  is  called  magnetic  induction. 

The  apparatus  for  this 

purpose   is   represented        p  FIG- 286- 

by  Fig.  286,  and  con-       ^  ^ 

sists  of  a  copper  wire 
coiled,  by  winding  it 
around  a  piece  of  wood. 
The  turns  of  the  wire 
should  be  close  together 
for  actual  experiment,  Electrical  Helix. 

they  being  parted  in  the 

figure  to  show  the  place  of  the  iron  to  be  magnetized.  The  best 
method  is,  to  place  the  coiled  wire,  which  is  called  an  electrical 
helix,  in  a  glass  tube,  the  two  ends  of  the  wire,  of  course,  pro- 
jecting. Then  placing  the  body  to  be  magnetized  within  the 
folds,  send  the  galvanic  influence  through  the  whole  by  placing 
the  poles  of  the  battery  in  the  cups. 

351.  Steel  thus  becomes  permanently  magnetic,  the  poles, 
Kowever,  changing  as  often  as  the  fluid  is  sent  through  it  in  a 
contrary  direction.     A  piece  of  watch-spring  placed  in  the  helix, 
and  then  suspended,  will  exhibit  polarity,  but  if  its  position  be 
reversed  in  the  helix,  and  the  current  again  sent  through  it,  the 
north  pole  will  become  south.     If  one  blade  of  a  knife  be  put 
into  one  end  of  the  helix,  it  will  repel  the  north  pole  of  a  mag- 
netic needle,  and  attract  the  south ;  and  if  the  other  blade  be 
placed  in  the  opposite  end  of  the  helix,  it  will  attract  the  north 
pole,  and  repel  the  south,  of  the  needle. 

352.  TEMPORARY  MAGNETS. —  Temporary  magnets,  of  almost 
any  power,  may  be  made  by  winding  a  thick  piece  of  soft  iron 
with  many  coils  of  insulated  copper  wire1  and  passing  the  gal- 
vanic  influence  through  it. 

The  best  form  of  a  magnet  for  this  purpose  is  that  of  a  horse- 
shoe, and  which  may  be  made  in  a  few  minutes  by  heating  and 
bending  a  piece  of  cylinder  iron,  an  inch  or  two  in  diameter, 
into  this  form. 

350.  What  is  meant  by  magnetic  induction  1  Explain  Fig.  286.  What  is  the  figure 
called  ?  351.  Does  any  substance  become  permanently  magnetic  by  the  electrical 
helix  ?  How  may  the  poles  of  a  magnet  be  changed  by  the  helix  1  352.  How  may 
temporary  magnets  be  made  I 


THERMO-ELECTRICITY. 


369 


The  copper  wire  (bell  wire)  may  be  insulated  by  winding  it 
with  cotton  thread.  If  this  can  not  be  procured,  common  bon- 
net wire  will  do,  though  it  makes  less  powerful  magnets  than 
copper. 

353.  The  coils  of  wire  FIG.  287. 
may  begin  near  one  pole 

of  the  magnet  and  term- 
inate near  the  other,  as 
represented  by  Fig.  287, 
or  the  wire  may  consist 
of  shorter  pieces  wound 
over  each  other,  on  any 
part  of  the  magnet.  In 
either  case,  the  ends  of 
the  wire,  where  several 
pieces  are  used,  must  be 
soldered  to  two  strips  of 
tinned  sheet  copper,  for 
the  combined  positive 
and  negative  poles  of  the 

wires.     To  form  the  mag-  Temporary  Magnet. 

net,  these  pieces  of  cop- 
per are  made  to  communicate  with  the  poles  of  the  battery,  by 
means  of  cups  containing  mercury,  as  shown  in  the  figure,  01 
by  any  other  method. 

354.  The  effect  is  surprising,  for  on  completing  the  circuit 
with  a  piece  of  iron  an  inch  in  diameter,  in  the  proper  form,  and 
properly  wound,  a  man  will  find  it  difficult  to  pull  off  the  arma- 
ture from  the  poles ;  but  on  displacing  one  of  the  galvanic  poles, 
the  attraction  ceases  instantly,  and  the  man,  if  not  careful,  will 
fall  backward,  taking  the  armature  with  him.     Magnets  have 
been  constructed   in  this  manner,  which  would  suspend  ten 
thousand  pounds. 

THERMO-ELECTRICITY. 

355.  This  means  electricity  by  heat,  and  its  principles  will 
be  understood,  when  it  is  stated  that  if  any  two  metals  of  dif- 
ferent kinds  be  joined  together  and  then  heated,  a  current  of 
electricity  will  pass  from  one  to  the  other.     Thus,  if  two  wires 
of  a  few  inches  in  length,  German-silver  and  brass,  have  their 
ends  soldered  together,  and  the  junction  heated  with  an  alcohol 


353.  For  what  purpose  dire  the  ends  of  the  wires  to  be  soldered  to  pieces  of  cop- 
>r  1    355.  What  is  meant  by  thermo-electricity  1 

16* 


perl 


570  ELECTROTYPE. 

lamp,  or  by  other  means,  a  current  of  electricity  will  flow  from 
the  silver  to  the  brass,  which  may  be  detected  by  the  gal- 
vanometer, or  by  the  common  electrical  needle. 

356.  Composition  of  German-silver. — As  this  alloy  is  cheap, 
and  is  much  used  for  electrical  purposes,  we  give  its  proportions. 
In  100  parts,  it  consists  of  copper  50,  zinc  30,  and  nickel  20. 
This  alloy  is  a  positive  electric  to  all  other  metals  except  bis- 
muth, to  which  it  is  negative. 

FIG.  ssa 


yhermo- Electricity. 

Writers  give  a  great  variety  of  combinations  of  different 
metals,  with  the  amount  of  electrical  influence  indicated  by  each. 
Among  these,  that  shown  by  Fig.  288,  is  among  the  most  easily 
constructed  and  most  powerful.  It  consists  of  ten  strips  of 
German -silver,  and  as  many  of  brass,  rolled  thin  and  laid  on 
each  other  with  their  alternate  ends  soldered  together.  Strips 
of  pasteboard  are  placed  between  the  adjacent  metals,  so  that 
they  touch  only  at  the  ends  where  they  are  soldered.  Now  by 
heating  the  end  opposite  the  poles  with  a  spirit  lamp,  and  bring- 
ing the  poles  in  contact,  an  electrical  current  will  flow  from  one 
side  or  pole,  to  the  other,  in  the  direction  of  the  arrows. 

ELECTROTYPE. 

35  Y.  The  art  of  covering  the 'base  metals,  as  copper,  and  the 
alloys  of  zinc,  tin,  &c.,  with  gold  and  silver,  as  also  of  copying 
medals,  by  means  of  the  electrical  current,  is  called  electrotype 
or  voltatype. 

This  new  art  is  founded  on  the  simple  fact,  that  when  the 
galvanic  influence  is  passed  through  a  metallic  solution,  under 
certain  conditions,  decomposition  takes  place,  and  the  metal  is 
deposited  in  its  pure  form  on  the  negative  pole  of  the  battery. 

The  theory  by  which  this  effect  is  explained  is,  that  the 

356.  Explain  by  Fig.  288,  how  thermo-electricity  is  developed  ?  357.  What  is  elec- 
trotype 1  On  what  fact  is  it  said  this  art  is  founded?  On  which  pole  is  the  metal 
deposited  7  What  is  the  theory  by  which  this  effect  is  explained  1 


ELECTROTYPE.  37l 

hydrogen  evolved  by  the  action  of  the  acid  on  the  positive  pole 
of  the  battery  combines  with  the  oxygen  of  the  dissolved  metal 
foiming  water,  while  the  metal  itself  thus  set  free,  is  deposited 
at  the  negative  side  of  the  battery. 

Many  of  the  base  metals,  as  copper,  the  alloys  of  zinc,  and 
tin,  may  by  such  means  be  covered  with  gold,  or  silver,  and  thus 
a  cheap  and  easy  method  of  gilding  and  plating  is  effected. 

This  art,  now  only  a  few  years  old,  has  excited  great  interest, 
not  only  among  men  of  science,  but  among  mechanics,  so  that 
in  England  many  hundreds,  and  perhaps  thousands  of  hands 
are  already  employed  in  silvering,  gilding,  and  coppering,  taking 
impressions  of  medals  and  of  copperplates,  for  printing,  and  of 
performing  such  other  work  as  the  art  is  capable  of.  Volumes 
have  been  written  to  explain  the  different  processes  to  which 
this  art  is  applicable,  and  considering  its  recent  discovery  and 
the  variety  of  uses  to  which  it  is  already  applied,  no  doubt  can 
exist  that  it  will  finally  become  of  great  importance  to  the  world. 

In  this  short  treatise  we  can  only  introduce  the  pupil  to  the 
subject,  by  describing  a  few  of  the  most  simple  processes  of  the 
art  in  question,  and  this  we  hope  to  do  in  so  plain  a  manner, 
that  any  one  of  common  iigenuity  can  gild,  silver,,  or  copper, 
and  take  impressions  of  medals  at  his  leisure. 

358.  COPYING  OF  MEDALS. — This  new  art  has  been  applied 
very  extensively  in  the  copying  of  ancient  coins  and  medals, 
which  it  does  in  the  utmost  perfection,  giving  every  letter,  and 
feature,  and  even  an  accidental  scratch,  ex-actly  like  the  original. 
When  the  coin  is  a  cameo,  the  figures  or  letters  ^eing  raised,  it 
is  obvious  that  if  the  metal  be  cast  directly  upon  */,  the  medal 
will  be  reversed,  that  is,  the  figures  will  be  indented,  and  the 
copy  will  be  an  intaglio  instead  of  a  cameo.     To  remedy  this,  a 
cast,  or  impression  must  first  be  taken  of  the  medal,  on  which 
the  electrotype  process  is  to  act,  when  the  copy  will,  in  all  re- 
spects, imitate  the  original. 

There  is  a  variety  of  ways  of  making  such  casts,  according  to 
the  substance  used  for  the  purpose.  We  shall  only  mention 
plaster  of  Paris,  wax,  and  fusible  metal. 

359.  PLASTER  CASTS. — When  plaster  is  used,  it  must  be, 
what  is  termed  baked,  that  is,  heated,  so  as  to  deprive  it  of  all 
mofeture.     This  is  the  preparation  of  which  stereotype  casts  are 
made.     The  dry  powder  being  mixed  with  water  to  the  con- 
sistence of  cream,  is  placed  on  the  medal  with  a  knife  to  the 
thickness  of  a  quarter  or  half  an  inch,  according  to  its  size.     In 
a  few  minutes  the  plaster  sets,  as  it  is  termed,  or  becomes  hard. 


372  ELECTROTYPE. 

To  insure  its  easy  detachment,  the  medal  is  rubbed  over  with  a 
little  oil. 

The  cast  thus  formed  is  first  to  be  coated  with  boiled  linseed 
oil,  and  then  its  face  covered  with  fine  pulverized  black  lead, 
taking  care  that  the  indented  parts  are  not  filled,  nor  the  raised 
parts  left  naked.  The  lead  answers  the  purpose  of  a  metallic 
surface,  on  which  the  copper  is  deposited  by  the  galvanic  current. 
This  is  a  curious  and  very  convenient  discovery,  since  wood  cuts, 
engraved  stones,  and  copies  in  sealing-wax,  can  thus  be  copied. 

To  insure  contact  between  the  black  lead  on  the  face  of  the 
cast  and  the  wire-conductor,  the  cast  is  to  be  pierced  with  an 
awl,  on  one  of  its  edges,  and  the  sharp  point  of  the  wire  passed 
to  the  face,  taking  care,  after  this  is  done,  to  rub  on  more  lead, 
so  that  it  shall  touch  the  point  of  the  wire,  and  thus  communi- 
cate with  the  whole  face  of  the  medal. 

360.  WAX  CASTS. — To  copy  medallions  of  plaster  of  Paris, 
place  the  cast  in  warm  water,  so  that  the  whole  may  be  satura- 
ted with  the  water,  but  keeping  the  face  above  it.     When  the 
cast  has  become  warm  and  moist,  remove,  and  having  put  a 
slip  of  paper  around  its  rim,  immediately  pour  into  the  cup  thus 
formed  bees  wax,  ready  melted  for  this  purpose.     In  this  way 
copies  may  be  taken,  not  only  from^)laster  casts,  but  from  those 
of  other  substances. 

To  rer  ler  the  surface  of  the  wax  a  conductor  of  electricity,  it 
is  to  he  covered  with  black  lead  in  the  manner  directed  for 
plaster  casts.  This  is  put  on  with  a  soft  brush,  until  it  becomes 
black  and  shining. 

The  electrical  conductor  is  now  to  be  heated  and  pressed  upon 
the  edge  of  the  wax,  taking  care  that  a  little  of  its  surface  is  left 
naked,  on,  and  around  which  the  black  lead  is  again  to  be 
rubbed,  to  insure  contact  with  the  whole  surface. 

Both  of  the  above  preparations  require  considerable  ingenuity 
and  attention,  in  order  to  make  them  succeed  in  receiving  the 
copper.  If  the  black  lead  does  not  communicate  with  the  pole, 
and  does  not  entirely  cover  the  surface,  or  if  it  happens  to  be  a 
poor  quality,  which  is  common,  the  process  will  not  succeed; 
but  patience,  and  repeated  trials,  with  attention  to  the  above 
descriptions,  will  insure  final  success. 

361.  FUSIBLE  METAL  CASTS. — This  alloy  is  composed  of  8 
parts  of  bismuth,  5  of  lead,  and  3  of  tin,  melted  together.     It 
melts  at  about  the  heat  of  boiling  water,  and  hence  may  be 
used  in  taking  casts  from  engraved  stones,  coins,  or  such  other 
substances  as  a  small  degree  of  heat  will  not  injure. 


ELECTROTYPE.  .  373 

To  take  a  cast  with  this  alloy,  surround  the  edge  of  the  medal 
to  be  copied,  with  a  slfp  of  paper,  by  means  of  paste,  so  as  to 
form  a  shallow  cup,  the  medal  being  the  bottom.  Then  hav- 
ing melted  the  alloy  in  a  spoon,  over  an  alcohol  lamp,  pour  it 
in,  giving  it  a  sudden  blow  on  the  table,  or  a  shake,  in  order  to 
detach  any  air,  which  may  adhere  to  the  medal.  In  a  minute 
or  two  it  will  be  cool,  and  ready  for  the  process. 

Another  method  is,  to  attach  the  medal  to  a  stick,  with  seal- 
ing-wax, and  having  poured  a  proper  quantity  of  the  fused  alloy 
on  a  smooth  board,  and  drawn  the  edge  of  a  card  over  it,  to 
take  off  the  dross,  place  the  medal  on  it,  and  with  a  steady 
hand  let  it  remain  until  the  cast  cools. 

Next,  having  the  end  of  the  copper  wire  for  the  zinc  pole 
clean,  heat  it  over  a  lamp,  and  touch  the  edge  of  the  cast  there- 
with, so  that  they  shall  adhere,  and  the  cast  will  now  be  ready 
for  the  galvanic  current. 

To  those  who  have  had  no  experience  in  the  electrotype  art, 
this  is  much  the  best,  and  most  easy  method  of  taking  copies, 
as  it  is  not  liable  to  failure  like  those  requiring  the  surfaces  of 
the  molds  to  be  black  leaded,  as  above  described. 

362.  GALVANIC  ARRANGEMENT. — Having  prepared  the  molds, 
as  above  directed,  these  are  next  to  be  placed  in  a  solution  ot 
the  sulphate-  of  copper,  (blue  vitriol)  and  subjected  to  the  elec- 
trical current.  For  this  purpose  only  a  very  simple  battery  is 
required,  especially  where  the  object  is  merely  a  matter  of 
curiosity. 

For  small  experiments,  a  glass  jar  holding  a  pint,  or  a  pitcher, 
or  even  a  tumbler  will  answer,  to  hold  the  solution.  Provide 
also  a  cylinder  of  glass  two  inches  in  diameter,  and  stop  the 
bottom  with  some  moist  plaster  of  Paris,  or  instead  thereof,  tie 
around  it  a  piece  of  bladder,  or  thin  leather,  or  the  whole  cylin- 
der may  be  made  of  leather,  with  the  edges  sewed  nicely  to- 
gether, and  stopped  with  a  cork,  so  that  it  will  not  leak.  The 
object  of  this  part  of  the  arrangement  is,  to  keep  the  dilute  sul- 
phuric acid  which  this  contains,  from  mixing  with  the  solution  of 
sulphate  of  copper,  which  surrounds  it,  still  having  the  texture 
of  this  vessel  so  spongy  as  to  allow  the  galvanic  current  to  pass 
through  the  moisture  which  it  absorbs,  water  being  a  good  con- 
ductor of  electricity. 

Provide  also  a  piece  of  zinc  in  form  of  a  bar,  or  cylinder,  or 
slip,  of  such  size  as  to  pass  freely  into  the  above  described 
cylinder. 

Having  now  the   materials,  the   arrangement  will  readily 


374 


SMEE'S  BATTERY, 


FIG.  289. 


be  understood  by  Fig.  289,  where  c  is  the  ves- 
sel containing  the  solution  of  sulphate  of  cop- 
per ;  a,  the  cylinder  of  leather,  or  glass ;  2,  the 
zinc,  to  which  a  piece  of  copper  wire  is  fastened, 
and  at  the  other  end  of  which,  is  the  cast  m,  to  be 
copied.  The  proportions  for  the  vessel,  a,  are 
about  1  part  sulphuric  acid  to  16  of  water  by 
measure.  The  solution  of  copper  for  c,  may  be  in 
the  proportions  of  2  ounces  of  the  salt  to  4  ounces 
of  water.  The  voltaic  current  passes  from  the 
positive  zinc  to  the  negative  amalgam  cast,  where 
the  pure  copper  is  deposited. 

In  order  to  keep  the  solution  saturated,  a  little 
sulphate  of  copper  is  tied  in  a  rag,  and  suspended  in  the  solu- 
tion. In  24  or  36  hours,  the  copper,  (if  all  is  right,)  will  be 
sufficiently  thick  on  the  cast,  the  back  and  edges  of  which  should 
be  covered  with  varnish  to  prevent  its  deposition  except  on  the 
face. 

If  the  copper  covers  the  edges,  a  file  or  knife  will  remove  it, 
when  by  inserting  the  edge  of  the  knife  between  the  two  metals, 
the  copy  will  be  separated,  and  will  be  found  an  exact  copy  of 
the  original. 

If  the  acid  in  the  inner  cylinder  is  too  strong,  the  process  is 
often  too  vigorous,  and  the  deposition,  instead  of  being  a  film 
of  solid  copper  on  the  cast,  will  be  in  the  form  of  small  grains 
on  the  lower  end  of  the  wire.  The  weakest  power  consistent 
with  precipitation  should  therefore  be  applied. 


BMEE7S   BATTERY. 


363.  This  is  an  improved  method  of  copying  casts,  or  molds, 
in  copper.  It  consists  of  two  glass  vessels,  each  holding  a  pint, 
or  less,  one  of  which  holds  the  battery,  and  the  other  the  de- 
positing apparatus.  These  arrangements  will  be  understood 
by  Fig.  290,  of  which  1  is  a  little  mercury  on  the  bottom  of 
the  vessel,  containing  the  battery.  Just  above  this  is  a  piece 
of  platinum  foil,  suspended  in  the  center.  A  piece  of  zinc,  4, 
rests  against  the  side  of  the  vessel.  A  curved  copper  wire,  3, 
descends  through  the  liquid,  insulated  by  a  glass  tube.  This 
wire,  by  the  mercury,  connects  the  zinc  plate  with  the  metallic 
cup  on  the  top  of  the  jar,  and  by  the  wire,  2,  with  the  other 
jar.  The  wire,  5,  descends  from  the  screw-cup  into  the  depos- 
iting cell,  to  the  end  of  which  the  cast,  6,  is  suspended.  The 
plate  7,  is  a  piece  of  copper  suspended  in  the  solution  of  sulphate 


ELECTRO-MAGNETISM. 


375 


of  copper,  in  order  to  keep 
it  always  of  the  same 
strength,  a  portion  being 
dissolved,  while  another 
portion  is  deposited  on 
the  cast. 

The  liquid  in  the  bat- 
tery is  composed  of  one 
part  sulphuric  acid,  and 
20  or  30  of  water.  That 
in  the  depositing  side,  is 
composed  of  2  ounces  of 
sulphate  of  copper,  1  ounce 

of  sulphuric  acid,  and    15    •  Smee's  Battery. 

ounces  of  water. 

The  general  directions  for  obtaining  casts  have  been  given 
above,  and  need  not  be  repeated. 


MAGNETISM    BY    ELECTRO-MAGNETISM. 


364.  The  apparatus,  Fig.  291,  is  designed  to  communicate 
strong  and  permanent  magnetism  to  steel.  It  consists  of  a 
small  Smee's  battery,  with  its  opposite  poles  connected  with  the 
horizontal  U  magnet,  which  is  closely  wound  with  insulated 
copper  wire.  Of  course  the  wires  convey  the  electrical  influence 
from  the  positive  to  the  negative  sides  of  the  battery. 


FIG.  291. 


Magnetism  by  Electro-Magnetism. 


The  cut  represents  a  U  magnet  in  the  process  of  being  mag- 
netized. This  is  done  by  drawing  it  from  the  bend,  across  the 
electro -magnet  to  the  poles,  and  repeating  this  on  both  its  sides, 
taking  care  to  do  it  in  the  same  direction.  A  steel  bar  may  be 


376  ELECTRO-GILDING. 

magnetized  by  the  same  process,  or,  if  a  short  one,  by  applying 
it  as  an  armature  to  the  poles  of  the  electro-magnet ;  the  north 
pole  becoming  the  south  pole  of  the  new  magnet. 

365.  To  remove  the  magnetism  of  a  steel  magnet  of  the  U 
form,  it  is  only  required  to  reverse  the  process,  that  is,  to  place 
one  of  its  poles  on  each  pole  of  the  electro-magnet,  and  draw  it 
over  them,  in  the  direction  contrary  to  the  indication  of  the 
arrow  seen  in  the  figure. 

In  the  vertical  magnet,  the  letters  N  S,  indicate  its  north  and 
south  poles. 


ELECTRO-GILDING. 


366.  Gilding  without  a  Battery. — After  the  solution  is  pre- 
pared, the  process  of  electrotype-gilding  is  quite  simple,  and  may 
be  performed  by  any  one  of  common  ingenuity. 

The  solution  for  this  purpose  is  cyanide  of  gold  dissolved  in 
pure  water.  This  is  prepared  by  dissolving  the  metal  in  aqua- 
regia,  composed  of  one  part  nitric,  and  two  of  muriatic  acid. 
Ten  or  fifteen  grains  of  gold,  to  an  ounce  and  a  half  of  the 
aqua-regia  may  be  the  proportions.  The  acid  being  evaporated, 
the  salt  which  is  called  the  chloride  of  gold  is  dissolved  in  a 
solution,  made  by  mixing  an  ounce  of  the  cyanuret  of  potash 
with  a  pint  of  pure  water.  The  cyanuret  of  potash  is  decom- 
posed and  a  cyanide  of  gold  remains  in  solution.  About  20 
grains  of  the  chloride  of  gold  is  a  proper  quantity  for  a  pint  of 
the  solution.  The  cyanuret  of  potash,  and  the  chloride,  or 
oxyd  of  gold,  may  be  bought  at  the  apothecaries. 

Having  prepared  the  solution,  the  most  simple  method  of 
gilding  is  to  pour  a  quantity  of  it  into  a  glass  jar,  or  a  tumbler, 
and  place  in  it  the  silver,  copper,  or  German-silver  to  be  gilded, 
in  contact  with  a  piece  of  bright  zinc,  and  the  process  will  im- 
mediately begin.  No  other  battery,  except  that  formed  by  the 
zinc,  and  metal  which  receives  the  gold,  is  required.  The  zinc 
at  the  point  of  contact  must  be  bright  and  well  fastened  to  the 
other  metal  by  a  string  or  otherwise.  The  process  will  be 
hastened  by  warmth,  which  may  be  applied  by  placing  the  jar 
and  its  contents  in  a  vessel  of  warm  water.  So  far  as  the  author 
knows,  this  simple  process  originated  with  himself,  and  answers 
admirably  as  an  experiment  in  the  electrotype  art.  The  gold, 
however,  is  apt  to  settle  upon  the  zinc,  but  which  may  be  pre- 
vented by  a  little  shellac  varnish  rubbed  on  it,  except  at  the 
point  of  contact.  The  handles  of  scissors,  silver  spectacles,  pen- 
cils, &c.,  may  be  handsomely  gilt  by  this  process. 


ELECTRO-PLATING.  37  7 

367.  GILDING  WITH  A  BATTERY. — If  the  operator  desires  to 
extend  his  experiments  in  the  art  of  electro -gilding,  a  small  bat- 
tery must  be  employed,  of  which,  there  are  many  varieties. 
The  best  for  more  extensive  operations,  is  that  composed  of 
platinized  silver,  and  amalgamated  zinc. 

For  this  purpose  the  platina  is  first  dissolved  in  aqua-regia, 
in  proportion  of  10  grains  to  the  ounce,  and  then  precipitated 
on  the  silver.  The  silver  is  in  sheets,  such  as  is  used  for  plating, 
no  thicker  than  thin  writing  paper.  This  may  be  obtained  of 
the  silver- platers,  and  being  well  cleaned,  is  ready  for  the  process. 

These  plates  being  covered  with  platina,  are  insoluble  in  the 
acid  employed,  and  hence  they  will  last  many  years.  The  amal- 
gamated plates  are  also  durable,  and  do  not  require  cleaning. 

368.  These  platinized  sheets  ar^ confined  between  two  plates 
of  amalgamated  zinc.     The  process  of  amalgamation  consists  in 
rubbing  mercury,  with  a  little  mass  of  cotton  wool  held  in  the 
fingers,  on  the  clean  zinc.     These  plates  may  be  fixed  half  an 
inch  apart  by  means  of  little  pieces  of  wood,  with  the  sheets  be- 
tween them,  but  not  touching  each  other.     The  plates,  having 
a  metallic  connection,  form  the  positive  side  of  the  battery, 
while  a  copper  wire  soldered  to  the  silver  sheet  makes  the  nega- 
tive side.     The  dimensions  of  these  plates  may  be  four  or  five 
inches  long,  and  three  or  four  wide. 

For  experimental  purposes,  however,  a  less  expensive  battery 
may  be  used,  that  represented  by  Fig.  289,  made  of  copper  and 
zinc,  being  sufficient. 

To  gild  by  means  of  a  battery,  place  the  solution,  made  as 
above  described,  in  a  glass  vessel,  and  connect  the  article  to  be 
gilded  with  the  pole  coming  from  the  zinc  side  of  the  battery, 
letting  the  other  wire,  which  should  be  tipped  with  a  little  piece 
of  gold,  dip  into  the  solution.  The  gilding  process  will  imme- 
diately begin,  and  in  three  or  four  hours  a  good  coat  of  gold 
will  be  deposited  on  the  article  immersed. 

To  keep  the  solution  quite  pure,  the  tips  of  the  poles  where 
they  dip  into  the  fluid  should  be  of  gold.  If  they  are  of  copper, 
a  portion  of  the  metal  will  be  dissolved  and  injure  the  result. 

ELECTRO-PLATINO. 

369.  The  process  of  silvering  copper,  or  the  alloys  of  the 
metals,  such  as  German-silver,  is  done  on  the  same  principle  as 
that  described  for  gilding,  but  there  seems  to  be  more  difficulty 
in  making  the  process  succeed  to  the  satisfaction  of  the  artist 
than  there  is  in  depositing  gold. 


378  ELECTRO -PLATING. 

The  following  is  the  method  employed  by  Mr.  Sumner  Smith, 
of  this  city,  the  most  experienced  electrotype  artist  within  our 
acquaintance.  It  will  succeed  perfectly  in  the  hands  of  those 
who  will  follow  the  directions. 

Make  a  solution  of  cyanuret  of  potash  in  pure  water,  in  the 
proportion  of  an  ounce  to  a  pint.  Having  placed  it  in  a  glass 
vessel,  prepare  the  battery  for  action  as  usual.  Then  attach  to 
the  pole  of  the  silver,  or  copper  side  of  the  battery,  a  thin  plate 
of  silver,  and  immerse  this  in  the  cyanuret  solution.  The  pole 
from  the  zinc  side  being  now  dipped  into  the  fluid,  the  electro- 
chemical action  on  the  silver  plate  instantly  begins,  and  a  rapid 
decomposition  of  the  metal  is  effected,  and  in  a  short  time  the 
solution  will  be  saturated  with  the  silver,  as  will  be  indicated 
by  the  deposition  of  the  metal  on  the  end  of  the  copper  pole 
coming  from  the  zinc  side  of  the  battery.  The  solution  is  now 
ready  for  use,  but  the  remains  of  the  silver,  still  undissolved, 
must  not  be  removed  before  immersing  the  articles  to  be  plated, 
since  the  solution  is  thus  kept  saturated. 

This  solution  is  much  better  than  that  prepared  by  dissolving 
the  silver  separately  in  an  acid,  and  then  re-dissolving  in  the 
cyanuret  of  potash  as  is  usually  done,  for  in  the  latter  case  the 
silver  is  apt  to  be  deposited  on  German-silver,  brass,  iron,  and 
other  metals,  without  the  galvanic  action,  in  which  case  it  does 
not  adhere  well,  whereas  the  solution  made  as  above  directed 
is  not  liable  to  this  imperfection. 

During  the  preparation  of  the  fluid,  only  a  very  small  copper 
wire  should  be  employed  on  the  zinc  side  of  the  battery. 

The  articles  to  be  plated  must  be  well  cleaned  before  immer- 
sion. To  effect  this,  dip  them  into  dilute  sulphuric  acid  for  a 
few  minutes,  then  rub  them  with  sand  or  whiting,  and  rinse  in 
pure  water. 

Now  having  exchanged  the  small  copper  pole  of  the  zinc  side 
of  the  battery,  for  a  larger  one  of  the  same  metal,  tipped  with 
silver,  connect  the  article  to  be  plated  with  this,  the  other  pole 
with  the  silver  plate  attached  being  still  immersed  in  the  solution. 

The  process  must  now  be  watched,  and  the  silver  attached  to 
the  copper  side  raised  nearly  out  of  the  fluid,  in  case  bubbles 
of  hydrogen  are  observed  to  rise  from  the  pole  on  the  other 
side,  or  the  articles  attached  to  it.  The  greater  the  surface  of 
silver  in  the  fluid,  the  more  energetic  will  be  the  action,  short 
of  the  evolution  of  hydrogen  from  the  other  pole,  but  when  this 
is  observed,  the  decomposing  silver  must  be  raised  so  far  out  of 
the  fluid  as  to  stop  its  evolution. 


ERICSSON'S  CALORIC  ENGINE. 


379 


By  this  method  a  thick  and  durable  coat  of  silver  may  be 
placed  on  old  copper  tea-pots,  candlesticks,  or  other  vessels  of 
this  sort,  where  the  silvering  has  been  worn  off  by  long  use. 

ERICSSON'S  CALORIC  ENGINE. 

Description  of  the  plates. — The  following  description  of  the 
plates,  and  explanation  of  the  working  of  the  caloric  engine, 
was  furnished  for  Appleton's  Magazine  by  Capt.  Ericsson  himself, 
from  which  this  is  abridged.  The  plates  are  an  exact  working 
drawing  of  the  stationary  test  engine,  that  being  more  available 
for  illustration  than  the  engines  of  the  ship. 

a,  Air  receiver,  b  6,  supply  cylinder ;  e',  self-acting  valve  for 
letting  air  into,  and  e'  e',  self-acting  valve  for  letting  air  out  of 
the  same,  c,  supply  piston ;  c',  piston-rod  of  the  same,  con- 


FIG.  292. 


Caloric  Engine. 

nected  to  the  working  beam  of  the  engine,  d  d,  working  cylin- 
der ;  d'  d',  holes  at  the  junction  of  the  two  cylinders,  through 
which  the  atmospheric  air  passes  in  and  out  freely,  e  e,  work- 
ing piston ;  d"  d",  rods  connecting  the  two  pistons  together. 
e",  air-tight  vessel  suspended  below  the  working  piston,  filled 
with  clay  and  charcoal,  to  prevent  transmission  of  heat  from 
below.  //,  regenerator ;  /',  disks  of  wire-net  placed  vertically 
in  the  regenerator  box.  y,  valve  worked  by  the  engine  for  ad- 


S80  ERICSSON'S  CALORIC  ENGINE. 

mitting  air  into  the  regenerator  and  working  cylinder ;  A,  valve 
for  letting  the  air  out  of  the  same,  i  i,  pipe,  open  to  the  at- 
mosphere, for  carrying  off  the  air  after  having  passed  through 
the  engine ;  k,  fire-place.  I  m  n,  thermometers  inserted  to 
ascertain  the  temperature  within,  o,  working  beam,  p,  connect- 
ing-rod. <7,  crank,  s,  pipe  from  cylinder  to  receiver. 

Operation. — A  slow  fire  being  kept  up  at  k  for  about  two 
hours,  until  the  parts  within  the  brick-work  shall  have  become 
moderately  heated,  the  air  receiver  is  charged  with  air  by  means 
of  a  hand-pump ;  as  soon  as  the  internal  pressure  shall  have 
reached  about  six  pounds  to  the  square  inch,  effected  in  less 
than  two  minutes,  the  hand-pump  is  stopped,  and  the  valve  g 
opened  by  a  starting  lever ;  the  compressed  air  from  the  re- 
ceiver, thus  admitted  under  the  valve  #,  rushes  through  the 
partially  heated  wires  /'  into  the  working  cylinder,  forcing  its 
piston  e  upwards,  as  also  the  supply  piston  c,  by  means  of  the 
connecting-rods  d"  d" ;  the  atmospheric  air  contained  in  the 
upper  part  of  b  will,  by  this  upward  movement  of  the  supply 
piston,  be  forced  through  the  valve  e"  into  the  air  receiver ; 
when  the  working  piston  has  reached  three-fourths  of  the  full 
up-stroke,  the  valve  g  is  closed  by  the  engine ;  and  when  the 
piston  has  arrived  at  the  full  up-stroke,  the  valve  h  is  opened. 
A  free  communication  with  the  atmosphere  being  thereby  es- 
tablished, by  means  of  the  open  pipe  i  i,  the  air  under  the 
working  piston  passes  off,  and,  owing  to  the  removal  of  pressure 
under  the  working  piston,  it  will  instantly  begin  to  descend  by 
its  own  weight. 

The  heated  air  from  under  the  working  piston,  in  passing  off 
through  the  wires  /',  gives  out  its  caloric  to  the  same  so  effect- 
ually, that,  on  reaching  the  thermometer  m,  the  temperature 
never  exceeds  that  of  the  entering  air  at  I  by  more  than  30° ; 
on  the  other  hand,  the  cold  air  from  the  receiver,  in  circulating 
through  the  meshes  of  wires  in  its  passage  to  the  working  cyl- 
inder, becomes  so  effectually  heated  that,  on  passing  n,  its  tem- 
perature is  increased  to  upwards  of  450°,  when  the  machine  is 
in  full  operation. 

During  the  descent  of  the  supply  piston  c,  the  outlet  valve  e" 
remains  closed  by  the  pressure  from  the  receiver,  whilst  the  in- 
let valve  e'  is  kept  open  by  suction,  and  hence  that  &  fresh  quan- 
tity of  air  enters  the  supply  cylinder  at  each  down  stroke  of  its 
piston,  and  by  the  up-stroke  is  forced  into  the  receiver.  The 
regenerator  measures  26  inches  in  height  and  width  ;  each  disk 
of  wire-net  contains  676  superficial  inches,  and  the  net  has  10 
meshes  to  the  inch  ;  each  superficial  inch  therefore  contains  100 


CALORIC    ENGINE.  381 

meshes,  which,  multiplied  by  676,  gives  67,600  meshes  to  each 
disk ;  200  disks  being  employed,  it  follows  that  each  regenerator 
contains  13,520,000  meshes,  and,  consequently,  if  we  consider 


FIG 


Caloric  Engine. 

that  there  are  as  many  small  spaces  between  the  disks  as  there 
are  meshes,  we  shall  find  that  the  air  within  the  regenerator  is 
distributed  in  27,000,000  of  minute  cells. 

In  explanation  of  the  wonderful  efficiency  of  the  regenerator, 
it  may  be  stated  that  each  disk  contains  1140  feet  of  wire  in 
length,  and  each  regenerator  228,000  feet,  or41£  miles  of  wire; 


382  DAGUERREOTYPE. 

the  superficial  measurement  of  which  is  2014  square  feet,  which 
is  equal  to  the  entire  surface  of  four  steam-boilers,  forty  feet  long 
and  four  feet  diameter ;  and  yet  the  regenerator  displaying  that 
great  amount  of  heating  surface  is  only  a  two  foot  cube,  less 
than  -iViTo"  of  the  bulk  of  said  boilers. 

DAGUERREOTYPE. 

3*74.  This  branch  of  photography  was  the  invention  of  M. 
Daguerre,  an  ingenious  French  artist,  and  is  entirely  independ 
ent  of  the  art  of  taking  impressions  on  paper,  as  above  de- 
scribed. In  that  the  pictures  are  reversed,  in  this  they  are  in 
the  natural  position,  and  instead  of  paper,  the  picture  is  on 
silver. 

As  an  art,  this  is  one  of  the  most  curious  and  wonderful 
discoveries  of  the  present  age ;  for  when  we  witness  the  variety 
of  means  necessary  to  the  result,  it  would  appear  equally  im- 
probable that  either  accident  or  design  could  possibly  have 
produced  such  an  end  by  means  so  vario*us  and  complicated, 
and  to  which  no  other  art,  (save  in  the  use  of  the  camera 
obscura,)  has  the  least  analogy  in  the  manner  in  which  the  ob- 
ject is  accomplished. 

This  being  a  subject  of  considerable  public  interest,  and, 
withal,  a  strictly  philosophical  art,  we  shall  here  describe  all 
the  manipulations  as  they  succeed  each  other  in  producing  the 
result,  a  human  likeness. 

The  whole  process  may  conveniently  be  divided  into  eight 
distinct  operations.  1st.  Polishing  the  plate.  2d.  Exposing  it 
to  the  vapor  of  iodine.  3d.  Exposing  it  to  the  vapor  of  bromine. 
4th.  Adjusting  the  plate  in  the  camera  obscura.  5th.  Exposing 
it  to  the  vapor  of  mercury.  6th.  Removing  the  sensitive 
coating.  7th.  Gilding  the  picture.  8th.  Coloring  the  picture. 

1.  Polishing  the  Plate. — The  plates  are  made  of  thin  sheets 
of  silver,  plated  on  copper.  It  is  said  that  for  some  unknown 
reason  the  photographic  impression  takes  more  readily  on  these 
plates,  than  on  entire  silver.  The  silver  is  only  thick  enough 
to  prevent  reaching  the  copper  in  the  process  of  scouring  and 
polishing. 

The  polishing  is  considered  one  of  the  most  difficult  and  im- 
portant manipulations  in  the  art,  and  hence  hundreds  of  pages 
have  been  written  to  describe  the  various  methods  devised  and 
employed  by  different  artists  or  amateurs. 

We  can  only  state  here,  that  the  plate  is  first  scoured  with 
emery  to  take  off  the  impressions  of  the  hammer  in  plenish- 
ing ;  then  pumice,  finely  powdered,  is  used,  with  alcohol,  to 


DAGUERREOTYPE.  383 

remove  all  oily  matter,  and  after  several  other  operations,  it  is 
finally  given  the  last  finish  by  means  of  a  velvet  cushion  cov- 
ered with  rouge. 

2.  Iodizing  the  Plate. — After  the  plate  is  polished,  it  is  in- 
stantly covered  from  the  breath,  the  light,  and  the  air,  nor 
must  it  be  touched,  even  on  the  edges,  with  the  naked  hand ; 
but  being  placed  on  a  little  frame,  with  the  face  down,  it  is 
carried  to  a  box  containing  iodine,  over  which  it  is  placed  as  a 
cover.     Here  it  remains  for  a  moment  or  two  in  a  darkened 
room,  being  often  examined  by  the  artist,  whose  eye  decides 
by  the  yellowish  color  to  which  the  silver  changes,  the  instant 
when  the  metal  has  combined  with  the   proper   quantity  of 
iodine.     This  is  a  very  critical  part  of  the  process,  and  requires 
a  good  eye  and  much  experience.     The  vapor  of  iodine  forms 
a  film  of  the  iodid  of  silver  on  the  metal,  and  it  is  this  which 
makes  it  sensible  to  the  light  of  the  camera,  by  which  the  pic- 
ture is  formed.     If  the  film  of  iodine  is  too  thick,  the  picture 
will  be  too  deep,  and  dark ;  if  too  thin,  either  a  light  impres- 
sion, or  none  at  all,  will  be  made. 

3.  Exposure  of  the  Vapor  of  Bromine. — Bromine  is  a  pe-- 
culiar  substance,  in  the  liquid  form,  of  a  deep  red  color,  ex- 
ceedingly  volatile,  very  poisonous,  and  having  an  odor   like 
chlorine  and  iodine,  combined.     It  is  extracted  from  sea  water, 
and  the  ashes  of  marine  vegetables. 

This  the  photographic  artists  call  an  accelerating  substance, 
because  it  diminishes  the  time  required  to  take  the  picture  in 
the  camera  obscura. 

The  iodized  plate  will  receive  the  picture  without  it,  but  the 
sitter  has  to  remain  without  motion  before  the  camera  for  sev- 
eral minutes,  whereas  by  using  the  bromine,  the  impression  is 
given,  in  a  minute,  or  in  a  minute  and  a  quarter.  Now  as  the 
least  motion  in  the  sitter  spoils  the  likeness,  it  is  obvious  that 
bromine  is  of  much  importance  to  the  art,  especially  to  nervous 
people  and  children. 

The  bromine  is  contained  in  a  glass  vessel  closely  covered, 
and  is  applied  by  sliding  the  plate  over  it  for  a  few  seconds. 

4.  Adjusting  the  Plate  in  the   Camera. — The  plate  is  now 
ready  for  the  photographic  impression  by  means  of  the  camera. 
If  a  likeness  of  a  person  is  to  be  taken,  he  is  already  placed 
before  the  instrument,  in  a  posture  which  the  artist  thinks  will 
give  the  most  striking  picture,  and  is  told  that  the  only  motion 
he  can  make  for  a  half  a  minute  to  a  minute,  is  winking. 

The  artist  now  takes  the  plate  from   a  dark  box,  and  undej 


884  DAGUERREOTYPE. 

cover  of  a  black  cloth  fixes  it  in  the  focus  of  the  lens.  This  is 
done  in  a  light  room,  with  the  rays  of  the  sun  diffused  by 
means  of  white  curtains. 

The  artist  having  left  the  sitter  for  the  specified  time,  returns, 
and  removes  the  plate  for  the  next  operation.  Still,  not  the 
least  visible  change  has  taken  place  on  the  bright  surface  of  the 
silver.  If  examined  ever  so  nicely,  no  sign  of  a  human  face  is 
to  be  seen,  and  the  sitter  who  sees  the  plate,  and  knows  nothing 
of  the  art,  wonders  what  next  is  to  be  done. 

5.  Exposure  to  the  Fumes  of  Mercury. — The  plate  is  next 
exposed  to  the  fumes    of  mercury.     This  is  contained  in  an 
iron  box  in  a  darkened  room,  and  is  heated  by  means  of  an 
alcohol  lamp,  to  about  180  degrees,  Fah.     The  cover  of  the 
box  being  removed,  the  plate  is  laid  on,  with  the  silver  side 
down,  in  its  stead. 

After  a  few  minutes,  the  artist  examines  it,  and  by  a  faint 
light  now  sees  that  the  desired  picture  begins  to  appear.  It  is 
again  returned  for  a  few  minutes  longer,  until  the  likeness  is 
fully  developed. 

If  too  long  exposed  to  the  mercury,  the  surface  of  the  silver 
turns  to  a  dark  ashy  hue,  and  the  picture  is  ruined ;  if  re- 
moved too  soon,  the  impression  is  too  faint  to  be  distinct  to  the 
eye. 

6.  Removal  of  the  Sensitive  Coating. — The  next  operation 
consists  in  the  removal  of  the  iodine,  which  not  only  gives  the 
silver  a  yellowish  tinge,  but  if  suffered  to  remain,  would  darken, 
and  finally  ruin  the  picture.     Formerly   this   was  done  by  a 
solution  of  common  salt,  but  experiment  has  shown  that  the 
peculiar  chemical  compound  called  hyposulphate  of  soda,  an- 
swers the  purpose  far  better.     This  is  a  beautiful  transparent 
crystalized  salt,  prepared  by  chemists  for  the  express  purpose. 

A  solution  of  this  is  poured  on  the  plate  until  the  iodine  is 
entirely  removed,  and  now  the  picture,  for  the  first  time,  may 
be  exposed  to  the  light  of  the  sun  without  injury,  but  the  plate 
has  still  to  be  washed  in  pure  water,  to  remove  all  remains  of 
the  hyposulphate,  and  then  heated  and  dried  over  an  alcohol 
lamp. 

7.  Gilding  the  Picture. — This  is  called,  fixing,  by  the  chlo- 
ride of  gold. 

Having  washed  the  picture  thoroughly,  it  is  then  to  be  placed 
on  the  fixing  stand,  which  is  to  be  adjusted  previously,  to  a 
perfect  level,  and  as  much  solution  of  chloride  of  gold  as  the 
plate  can  retain,  poured  on.  The  alcohol  lamp  is  then  held 


MORSE'S    ELECTRO-MAGNETIC  TELEGRAPH.  385 

under  all  parts  of  it  successively.  At  first  the  image  assumes 
a  dark  color,  but  in  a  few  minutes  grows  light,  and  acquires  an 
intense  and  beautiful  appearance. 

The  lamp  is  now  removed,  and  the  plate  is  again  well  washed 
in  pure  water,  and  then  dried  by  heat. 

Before  gilding,  the  impression  may  be  removed  by  repolish- 
ing  the  plate,  when  it  is  perfectly  restored  ;  but  after  gilding,  no 
polishing  or  scouring  will  so  obliterate  the  picture,  as  to  make 
it  answer  for  a  second  impression.  Such  plates  are  either  sold 
for  the  silver  they  contain,  or  are  re-plated  by  the  electrotype 
process. 

8.  Coloring  the  Picture. — Coloring  daguerreotype  pictures 
is  an  American  invention,  and  has  been  considered  a  secret, 
though  at  the  present  time  it  is  done  with  more  or  less  success 
by  most  artists. 

The  color  consists  of  the  oxyds  of  several  metals,  ground  to 
an  impalpable  powder.  They  are  laid  on  in  a  dry  state,  with 
soft  camel-hair  pencils,  after  the  process  of  gilding.  The  plate 
is  then  heated,  by  which  they  are  fixed.  This  is  a  very  deli- 
cate part  of  the  art,  and  should  not  be  undertaken  by  those 
who  have  not  a  good  eye,  and  a  light  hand. 

The  author  is  indebted  to  Mr.  N.  G.  Burgess,  of  192  Broad- 
way, New  York,  for  much  of  the  information  contained  in  tho 
above  account  of  the  daguerreotype  art.  Mr.  B.  is  an  experi- 
enced and  expert  artist  in  this  line. 

MORSE'S    ELECTRO-MAGNETIC  TELEGRAPH. 

375.  The  means  by  which  Mr.  Morse  has  produced  his  won- 
der-working and  important  machine,  is  the  production  of  a 
temporary  magnet,  by  the  influence  of  the  galvanic  fluid. 

We  have  already  described  the  method  of  making  tempo- 
rary magnets  of  soft  iron,  by  covering  the  latter  with  insulated 
copper  wire,  to  each  end  of  which  the  poles  of  a  small  gal- 
vanic battery  is  applied. 

The  description  of  Fig.  2  87,  with  what  is  said  before  on  the 
subject,  will  inform  the  student  how  the  power  is  obtained  by 
which  the  philosopher  in  question  has  brought  before  the  world 
such  wonderful  and  unexpected  effects. 

The  machine  itself  is  sufficiently  simple,  and  will  be  compre- 
hended at  once,  by  those  who  have  made  electro-magnetic 
experiments,  by  the  annexed  diagram  and  description. 

The  temporary  magnet  A,  Fig.  2 94,  enveloped  with  its  insu- 
lated copper  wire,  is  fastened  to  the  wooden  frame  B  G,  by 
means  of  cords  or  otherwise. 

17 


386  MORSE'S  ELECTRO-MAGNETIC  TELEGRAPH. 

FIG.  294. 


Principle  of  Morse's  Telegraph. 

This  frame  also  supports  the  standard  H,  which  sustains  the 
revolving  drum  F,  on  which  the  paper  to  receive  the  emblem- 
atical alphabet  is  fixed,  M  being  the  edge  of  the  paper. 

To  the  arm  G,  is  appended  the  lever  C,  of  wood,  which  has 
a  slight  vertical  motion,  in  one  direction  by  the  steel  spring  D, 
and  in  the  other,  by  the  armature  of  soft  iron  E. 

The  two  poles  of  the  magnet  rest  in  two  little  cups  of  mer- 
cury, into  which  are  also  to  be  plunged  the  poles  of  the  mag- 
netic battery,  (not  shown  in  the  drawing,)  of  which  P  is  the 
positive,  and  N  the  negative.  The  steel  point  I,  attached  to  the 
lever,  is  designed  to  mark  the  telegraphic  alphabet  on  the  paper. 

Having  thus  explained  the  mechanism,  we  will  now  show  in 
what  manner  this  machine  acts  to  convey  intelligence  from  one 
part  of  the  country  to  another. 

It  has  already  been  explained  that  when  a  bar  of  soft  iron 
surrounded  by  insulated  copper  wire,  as  shown  at  A,  has  its  two 
poles  connected  with  the  poles  of  a  galvanic  battery,  the  iron 
instantly  becomes  a  magnet,  but  returns  to  its  former  state, 
or  ceases  to  be  magnetic,  the  instant  the  connection  between 
them  ceases. 

To  break  the  connection,  it  is  not  necessary  that  both  of  the 
poles  should  be  detached,  the  circuit  being  broken  by  the  sepa- 
ration of  one  only. 

Supposing  then,  that  N  and  P  are  the  poles  of  such  a  bat- 
tery, on  placing  N  into  the  cup  of  mercury,  the  wires  from  the 
soft  iron  being  already  there,  the  armature  E  is  instantly  at- 


VELOCITY    OF    ELECTRICITY.  387 

tracted,  which  brings  the  point  I  against  the  paper  on  the  re- 
volving wheel  F.  If  N  is  instantly  detached  after  the  point 
strikes  the  paper,  then  only  a  dot  will  be  made,  for  the  mag- 
netic power  ceasing  with  the  breaking  of  the  circuit,  the  spring 
D  withdraws  the  point  from  the  paper  the  instant  the  pole 
is  removed. 

If  a  line  is  required  in  the  telegraphic  alphabet,  then  the 
pole  is  kept  longer  in  the  vessel  of  mercury,  and  as  the  alphabet 
consists  of  dots,  and  lines  of  different  lengths,  it  is  obvious  that 
writing  in  this  manner  can  not  be  difficult  The  understanding 
of  the  alphabet  is  another  matter,  though  we  are  informed  that 
this  may  be  done  with  facility. 

The  marks  of  the  point  I,  are  made  by  indenting  the  paper, 
the  roller  on  which  it  is  fixed  being  made  of  steel  in  which  a 
groove  is  turned,  into  which  the  paper  is  forced  by  the  point. 
The  paper  is  therefore  raised  on  the  under  side  like  the  printing 
for  the  blind. 

The  roller  F  is  moved  by  means  of  clock-work,  having  an 
uniform  motion,  consequently  the  dots  and  lines  depending 
on  the  time  the  point  is  made  to  touch  the  paper,  are  always 
uniform. 

Now  with  respect  to  the  distance  apart  at  which  the  tem- 
porary magnet  and  writing  apparatus,  and  the  battery  are 
placed,  experiment  shows  that  it  makes  little  difference  with 
respect  to  time.  Thus,  suppose  the  battery  is  in  Hartford,  and 
the  magnet  in  New  York,  with  copper  or  iron  wires  reaching 
from  one  to  the  other.  Then  the  telegraphic  writer  at  Hartford, 
giving  the  signal  by  means  of  an.alarm  bell,  that  he  is  ready  to 
communicate,  draws  the  attention  of  the  person  at  New  York  to 
the  apparatus  there — the  galvanic  action  being  previously  broken 
by  taking  one  of  the  poles  from  the  battery  at  Hartford. 

If  now  we  suppose  the  letter  A  is  signified  by  a  single  dot, 
he  at  Hartford  dips  the  pole  in  the  cup  of  the  battery,  and  in- 
stantly at  New  York  the  soft  iron  becomes  a  magnet,  and  a  dot 
is  made  on  the  paper,  and  so,  the  rest  of  the  alphabet. 
.  The  wires  are  carried  through  the  air  by  being  wound  around 
glass  caps  supported  by  iron  L  shaped  arms,  which  are  driven 
into  wooden  posts  about  20  feet  from  the  ground.  These  posts 
are  erected  for  this  purpose  chiefly  on  the  railway  lines  from  50 
to  100  feet  apart. 


VELOCITY    OF    ELECTRICITY. 


376.  The  long  experience  of  the  officers  of  the  United  States 
government  on  the  coast  survey,  with  telegraphic  lines,  have 


388 


HOUSE'S    PRINTING    TELEGRAPH. 


enabled  them  to  measure  the  velocity  of  the  galvanic  current 
with  uncommon  accuracy.  From  experiments  and  calculations 
thus  made,  it  appears  that  its  velocity  is  about  fifteen  thousand 
four  hundred  miles  per  second. 

The  period  of  its  transit  between  Boston  and  Bangor,  was  re- 
cently measured,  and  the  result  was,  that  the  time  occupied  in 
its  passage,  was  the  one  hundred  and  sixtieth  of  a  second.  Ac- 
cording to  this  experiment  the  velocity  is  at  the  rate  of  16,000 
miles  per  second,  which  it  appears  is  about  600  miles  per  second 
more  than  the  estimates  made  on  the  coast  survey. — Annual 
Scientific  Discoveries. 

Telegraphs  in  the  Country. — According  to  a  recent  estimate, 
the  length  of  telegraphic  lines  in  the  country,  in  actual  opera- 
tion, is  not  far  from  15,000  miles. 

The  most  remote  points  in  communication  are  Quebec  and 
New  Orleans ;  their  distances  apart,  following  the  circuitous 
routes  of  the  wires,  being  about  3,000  miles. 

Number  of  Stations. — The  number  of  towns  and  villages  ac- 
commodated with  stations,  and  from  which,  therefore,  intelli- 
gence by  telegraph,  from  one  to  the  other,  or  from  one  to  all 
the  others  can  be  interchanged,  are  between  450  and  500. 


MORSE'S   TELEGRAPHIC    ALPHABET. 


Alphabet. 

A  -  — 

B 

C  --    - 

D 

E  - 

F 

G 

H 

I    -- 

K 

L  

M 


Alphabet. 


N 

o 
p 

Q 
R 
8 
T 
U 
V 

w 

X 
T 
Z 


Alphabet. 

&-      --- 


Numerals. 


HOUSE'S    PRINTING    TELEGRAPH. 


377.  This  instrument,  one  of  the  wonders  of  our  time,  prints 
all  communications  in  Roman  capitals,  and  that  much  more 
rapidly  than  the  most  expert  compositor. 

To  go  into  a  description  of  all  its  parts  would  probably  so 
confuse  the  mind  of  the  reader,  that  in  the  end  none  of  it  would 
be  understood.  We  shall,  therefore,  describe  only  such  portions 


r\ 


HOUSE'S    PRINTING 

of  the  machinery  as  are  necessary  to  show  'how  the  result  is 
produced. 

In  the  first  place,  when  a  communication  is  to  be  made  from 
one  city  to  another,  notice  is  given,  by  an  electrical  current  on 
the  wires,  which  occasions  a  vibration  of  a  part  of  the  ma- 
chinery, and  by  which  the  attendant  knows  that  a  message  is  to 
be  sent.  At  every  station  there  is  an  electrical  battery,  con- 
sisting of  12  or  14  cups,  the  power  most  commonly  used  being 
that  known  as  Grove's  battery,  a  description  of  which  may  be 
seen  in  another  place. 

The  forms  of  all  visible  parts  of  the  instrument  are  shown  by 
Fig.  295.  That  portion  by  which  the  printing  is  performed 
consists  of  a  soft  iron,  or  electro-magnet  contained  in  the  cylin- 
der A,  of  an  escapement  B,  moved  by  condensed  air,  by  means 
of  the  pump  G,  above  which  is  seen  the  band '  by  which  that 
part  of  the  machinery  is  turned ;  D  is  the  printing  apparatus, 
the  projecting  portion  being  the  lever ;  E  is  the  inking  band, 
by  which  the  type  are  inked  for  printing ;  F  is  a  strip  of  paper 
for  printing. 

FIG.  235 


House's  Printing  Telegraph. 

This  engine  is  moved  by  a  boy,  who  turns  the  wheel  by  the 
lever  shown,  and  by  which  air  is  condensed  by  the  pump  G,  and 
by  the  force  of  which,  the  printing  portion  of  the  machinery  is 
actuated. 


890  PRINTING    PRESS. 

The  action  of  the  electricity  in  this  telegraph  is  merely  to 
produce  a  correspondence  of  motion  in  the  machinery  at  the 
different  ends  of  the  line.  All  the  mechanical  results  are  pro- 
duced by  local,  mechanical  power,  connected  with  the  printing 
apparatus  at  each  station,  where  manual  force  is  employed  for 
this  purpose. 

378.  The  letters  on  the  keys,  moving  by  the  touch  like  those 
of  the  piano,  are  the  instruments  by  which  the  different  letters 
are,  one  by  one,  printed  from  one  station  to  the  next.  Thus 
one  letter  of  the  26,  on  the  different  keys,  will  be  printed  at. the 
other  end  of  the  line,  when  that  letter  is  depressed.  This  is 
done  by  converting  a  piece  of  soft  iron  into  a  magnet  at  the 
next  station,  on  the  principle  already  explained  and  illustrated, 
in  the  description  of  Morse's  telegraph,  only  that  the  letter, 
instead  of  the  point,  is  made  to  act  or  advance. 

This 'is  a  most  complicated  machine  as  a  whole,  though  its 
different  parts  are  sufficiently  simple.  The  effect,  though  the 
means  are  so  difficult  to  understand,  is  highly  curious  and  inter- 
esting, as  it  prints  Roman  capitals  at  the  rate  of  150  or  200  in 
a  minute.  This  is  done  on  strips  of  paper  an  inch  wide ;  and 
when  in  operation,  any  one  may  print  a  sentence,  as  his  own 
name,  by  touching  the  keys  on  which  the  letters  are  placed, 
which  spells  the  sentence. 


PRINTING   PRESS. 


3*78.  It  is  said  that  the  Chinese  printed  from  blocks  of  wood, 
with  letters  engraved  on  them,  before  the  Christian  era. 

But  the  first  printing  on  metallic  type,  was  executed  on  the 
celebrated  Mentz  Bible,  in  about  1450.  The  next  specimen  of 
printing  known  was  the  Psalter,  done  in  Germany,  m  1457. 

It  is  said  that  these  books  are  printed  in  such  a  style  of 
beauty  and  finish,  as  to  command  the  astonishment  of  all 
printers  who  behold  them,  and  that  even  at  the  present  day, 
with  all  our  boasted  inventions  and  improvements  in  the  arts, 
it  is  difficult  to  imitate,  and  hardly  possible  to  excel,  these  as 
specimens  of  work  in  the  art  of  printing. 

Of  the  mechanical  means  by  which  printing  has  been,  and 
still  is  performed,  many  singular  and  curious  examples  might 
be  described,  but  our  limits  will  only  admit  descriptions  of  two 
figures,  representing  Ramage's  press  and  the  cylinder  press. 

379.  Ramage's  Press. — This  press  was  that  most  commonly 
used  on  both  sides  of  the  Atlantic,  until  within  the  last  20  years. 
In  addition  to  this,  the  Stanhope  and  Smith  presses  were  used 


INKING    BALLS.  391 

in  England,  and  the  Clymer  and  Washington  in  this  country. 
These  may  be  considered  as  varieties  of  the  Ramage  ;  and  their 
description  would  possess  no  interest,  except  to  the  antiquated 
printer  who  had  worked  at  them  with  the  inking  balls,  now 
long  since  disused,  as  we  shall  see. 

The'  Ramage  press  is  repre-  FIG.  296. 

sented  by  Fig.  296,  and  will  be 
understood  by  the  following  de- 
scription :  The  cheeks  A  A,  are 
the  sides  of  the  wooden  frame 
which  supports  the  other  parts, 
and  sustains  the  force  of  the 
screw  by  which  the  impression 
is  made.  The  bed  B,  is  that 

part  on  which  the  type  are  laid  

for  printing.     The  ball  C  is  seen  Ramage's  Press. 

on  a  little  shelf,  called  the  rack, 

made  for  that  purpose.  [This  will  be  described  hereafter.] 
The  frisket,  F,  turns  down,  and  confines  the  sheet  on  the  tym- 
pan.  The  bar  or  lever  L,  turns  the  screw  by  which  the  force 
is  given  and  the  impression  on  the  type  made.  The  platen,  P, 
is  fastened  to  the  lower  end  of  the  screw,  being  the  part  by 
which  the  impression  is  made.  It  is  of  cast  iron,  about  two 
feet  square,  thick  at  the  center,  and  strong,  so  as  to  give  a 
heavy  force.  The  tympan,  T,  is  covered  writh  parchment  to  re- 
ceive the  sheet,  confined  by  the  frisket,  and  then  run  under  the 
platen  to  be  printed. 

Action. — The  type  being  ^et,  and  locked  firmly  in  an  iron 
frame,  called  a  form,  this  is  laid  on  the  bed,  and  the  type  inked 
by  the  balls ;  the  sheet  is  next  laid  on  the  tympan,  and  covered 
by  the  frisket,  which  has  open  spaces  for  the  pages,  as  seen  in 
the  figure.  The  type  and  sheet  spread  over  them,  are  then 
moved  under  the  platen  by  the  action  of  a  lever,  connected 
with  a  wooden  cylinder,  surrounded  by  leather  straps,  and  called 
the  rounce.  The  impression  is  then  made  by  pulling  the  lever,  by 
the  action  of  which,  on  the  screw,  the  platen  is  forced  upon  the 
paper,  and  this  on  the  type.  The  bed  is  then  "  run  out,"  the  type 
again  inked  by  dabbing  with  the  balls,  and  the  whole  is  again 
ready  to  be  run  in  for  another  impression,  and  so  on  to  the 
end. 


INKING    BALLS. 


380.  The  former  method  of  distributing  the  printing  ink  on 
the  type,  consisted  iu  the  use  of  a  pair  of  balls,  represented  by 


392 


INKING    ROLLER. 


FIG.  297. 


Inking  Balls. 


Fig.  297.  These  were  made  of 
sheeps'  skin,  undressed,  and  tech- 
nically called  pelts — were  six  or 
eight  inches  in  diameter,  stuffed 
with  wool,  and  furnished  with  wood- 
en handles. 

One  of  these  being  struck  on  the 
board  where  the  ink,  a  little  thicker 
than  cream,  was  spread,  took  up  a 
small  quantity,  which,  by  turning 
the  balls  skillfully  on  each  other, 
was  equally  spread  over  both.  They 
were  then  taken,  one  in  each  hand,  and  dabbed,  or  rapidly 
struck  on  the  type,  until  the  ink  was  nicely  distributed  over 
their  faces,  and  thus  they  were  made  ready  to  give  an  impres* 
sion.  This  was  a  critical  and  laborious  operation,  requiring 
much  experience  and  a  strong  arm,  like  that  of  a  blacksmith,  in 
order  to  cover  the  type  speedily  and  equally  with  the  ink. 
[Printer's  ink  is  made  of  oil  and  lampblack.] 

381.  Invention  of  the  Roller. — The  ancient  method  of  inking 
the  type,  as  above  described,  was  destined  to  give  place  to  an 
improvement,  which,  among  printers,  formed  an  era  long  to  be 
remembered. 

FIG.  298. 


Inking  Roller. 

Ihis  was  the  invention  of  the  roller  which  is  composed  of 
molasses,  glue,  and  tar,  intimately  mixed  and  combined  by  heat. 
This  composition  has  all  the  qualities  to  be  desired  for  this  pur- 
pose, namely,  softness,  elasticity,  and  readiness  to  receive  and 
impart  the  ink.  This  being  cast  into  a  cylinder,  on  a  wooden 
support,  and  fitted  to  an  iron  frame,  with  handles,  as  shown  by 
Fig.  298,  form  the  important  instrument  in  question. 

Rollers  have  also  been  made  of  India  rubber. 

As  the  ends  of  the  support  revolve  easily  in  the  frame,  all 
that  it  is  necessary  to  do  to  spread  the  ink  on  the  type,  is  first 


CYLINDER    PRESS.  393 

to  pass  the  roller  a  few  times  over  the  board  on  which  the  ink 
is  spread,  and  then  revolve  it  over  the  type  two  or  three 
times. 

This  invention  completely  obviated  the  most  laborious  and 
unpleasant  portion  of  the  art  of  printing  by  hand ;  and  in  ma- 
chine printing,  these  rollers  are  so  absolutely  indispensable,  that 
without  them  that  mode  of  printing,  without  which  the  world 
would  now  remain  in  comparative  ignorance,  would  have  to  be 
relinquished.  These  rollers  are  from  two  to  eight  inches  in 
diameter;  and  for  machine  work,  from  three  to  six  feet  in 
length. 

DOUBLE    CYLINDER   PRINTING    MACHINE. 

382.  This  printing  press,  when  compared  with  the  ancient 
or  former  one  of  Ramage,  already  described,  will  be  seen  to 
present  an  entirely  new  invention,  or  series  of  inventions ;  for 
many  years  were  consumed  in  devising  and  adapting  its  several 
parts  to  each  other,  and  bringing  it  to  the  state  of  perfection  in 
which  it  now  exists. 

Instead  of  printing,  as  did  the  hand  presses  in  old  times, 
2,000  copies  a  day,  by  means  of  ten  hour's  hard  labor  of  two 
men,  this  engine,  driven  by  steam,  will,  with  the  help  of  two 
boys  to  fix  the  sheets  in  their  places,  print  from  3,000  to  6,000 
sheets  per  hour,  or  from  30,000  to  60,000  copies  per  day. 
Such  are  the  improvements  in  printing  machines  within  the  last 
twenty  years. 

283.  Description. — This  is  a  length,  or  side  view  of  the  ma- 
chine ;  the  length  of  the  printing  cylinders  and  inking  rollers 
being  about  four  feet.  The  length  here  shown  of  the  whole 
machine,  is  from  8  to  10  feet,  and  the  height  to  the  upper 
cylinder  4  feet. 

The  ink,  about  the  consistence  of  cream,  is  taken  from  the 
trough,  which  is  of  the  length  of  the  small,  rapidly  revolving 
roller,  by  which  it  is  taken  up,  and  from  it  is  taken  by  adhe- 
sion  to  another  and  larger  roller,  from  which  it  is  derived  by 
the  type,  over  which  it  passes  with  a  reciprocating  motion. 

At,  or  during  each  impression,  the  ink  on  the  type  is  re- 
newed by  the  continually  revolving  rollers.  Thus,  while  this 
engine  is  in  action,  being  generally  moved  by  steam,  nothing 
more  is  necessary  than  to  supply  the  ink  by  putting  it  in  the 
trough,  and  to  place  the  ends  of  the  sheets  under  the  revolving 
cylinders,  which  latter  work  is  done  by  two  boys,  as  shown  by 
the  cut. 

17* 


SHARP'S  RIFLE.  395 

384.  The  parts  of  the  press  shown  by  the  Fig.  299,  are 
marked  as  follows :  The  bed  A,  on  the  left,  corresponds  to  the 
same  part  in  the  hand  press  already  explained.  This  has  a  re- 
ciprocating or  in  and  out  motion ;  the  type  which  rest  on  it, 
being  alternately  run  out  to  be  inked,  and  run  in  to  be  printed. 
The  revolving  cylinders  B  B,  receive  the  paper  and  press  it 
upon  the  type,  by  which  it  is  almost  instantly  printed.  The 
cam  C  moves  the  flies  D  D,  by  which  the  printed  sheets  are 
carefully  laid  away  in  a  pile.  This  movement  is  communicated 
by  the  cam  to  the  flies,  by  the  long  iron  bar  seen  oh  the  left. 
The  pulley  E,  moved  by  a  .strap  connected  with  the  steam 
power,  gives  motion  to  the  entire  machine  by-means  of  gearing. 
The  revolving  wheels  G  G,  give  motion  to  the  cylinders  and 
inking  rollers.  The  tape  wheels,  so  called,  H  H,  are  the  wheels 
over  which  run  tape  bands,  not  shown,  which  convey  the  printed 
sheets  from  the  form  to  the  flies.  The  printed  sheets  shown  at 
I,  have  been  laid  off  by  the  flies,  and  are  ready  for  circulation, 
or  the  bindery,  as  the  case  may  be. 

[The  author  has  thus  tried  his  best  to  give  an  idea  of  print- 
ing presses  to  those  who  never  saw  them ;  but  he  would  advise 
all  those  who  desire  to  know  how  printing  is  done,  especially  by 
a  cylinder  press,  to  go  and  see  with  their  own  eyes,  which  they 
can  do  now  in  nearly  every  village  in  the  country.] 


SHARP  S    RIFLE. 


385.  This  is  undoubtedly  for  the  purpose  designed,  the  most 
perfect  and  efficient  single  instrument  of  destruction  ever  in- 
vented ;  and  of  which,  we  here  propose  to  give  such  a  descrip- 
tion, with  illustrations,  as  to  make  all  its  peculiarities  readily 
understood. 

The  barrel  is  about  22  inches  long,  and  the  bore  of  the  size 
to  admit  round  balls  of  32  to  the  pound  ;  but  being  elongated, 
or  acorn-shaped,  the  number  is  only  18  to  the  pound. 

This  rifle  loads  at  the  breech,  the  form  of  the  ball  inclosed  in 
its  cartridge  being  shown  at  A,  Fig.  300,  introduced  into  its 
place. 

The  slide  B,  which  takes  the  place  of  the  breech  pin  in  other 
guns,  is  a  solid  piece  of  steel,  represented  depressed  for  the  in- 
troduction of  the  ball.  The  cone  E,  is  that  part  on  which  the 
percussion  cap,  or  its  substitute,  is  exploded,  and  which  in- 
flames the  charge  in  the  gun. 

The  manner  in  which  the  breech  slide  is  depressed,  will  be 
understood  by  the  section,  Fig.  301,  where  D  is  the  lever  by 


396 

FIG.  SCO. 


Sharp's  Rifle. 


which  it  is  drawn  down  for  the  introduction  of  the  ball,  and 
then  elevated  preparatory  to  the  discharge.  The  upper  and 
anterior  portion  of  the  slide,  has  a  cutting  edge,  seen  above  B, 
Fig.  298,  which  separates  the  end  of  the  paper  cartridge,  thus 
exposing  the  powder  to  the  action  of  the  percussion  priming,  by 
which  it  is  inflamed  and  the  gun  discharged. 

386.  The  Priming. — The  former  mode  of  discharging  this 
rifle  was  by  means  of  Maynard's  patent  priming,  which  con- 
sisted of  kernels  of  percussion  powder,  inclosed  in  varnished 
paper.  But  this  mode  the  inventor  of  the  rifle  found  objec- 
tionable on  several  accounts,  and  especially  as  it  became  useless 
on  exposure  to  moisture. 

He  therefore  invented  a  new,  and  an  entirely  original  mode 
of  prming,  which  has  been  recently  patented,  and  which  he 
has  allowed  the  author  to  figure,  and  explain  for  the  use  of  this 
work. 

This  consists  of  the  tube  A,  Fig.  300,  of  iron,  about  the  one- 
fifth  of  an  inch  in  diameter  and  two  inches  long,  called  the 
magazine.  In  the  lower  part  of  this  is  a  spring,  above  which 
are  the  priming  discs,  consisting  of  thin,  round  envelopes  of  cop- 
per, containing  the  percussion  powder,  completely  protected 
from  moisture,  so  that  they  may  remain  under  water  for  hours, 
or  weeks,  without  damage. 

Each  tube  holds  60  of  these  primers,  one  of  which  is  forced 
up  against  the  slide  C  by  the  spring.  When  the  hammer  is 
drawn  to  the  back  notch,  the  slit  B,  working  on  the  arm  of  the 
slide  C,  which  is  fastened  to  the  plate  of  the  lock,  draws  it 
back  from  over  the  tube  A,  and  admits  one  of  the  percussion 
discs  in  front  of  the  slide  at  C,  and  by  which,  when  the  trigger 
is  pulled,  it  is  thrown  forward,  between  the  face  of  the  hammer 


397 

FIG.  301. 


Sharp's  Rifle. 

and  the  cone,  where  it  is  instantly  exploded,  and  the  rifle  dis- 
charged. 

One  of  the  most  singular  and  curious  results  of  this  mechan- 
ism, is,  that  the  percussion  disc  is  struck,  as  it  were,  "  on  '  the 
wing,"  or  while  it  is  flying  between  the  hammer  and  the  cone; 
and  yet  it  never  fails  to  explode  in  the  proper  place  and  dis- 
charge the  gun,  let  its  position  be  vertical  or  horizontal. 

387.  Practical  effects  of  this  Rifle. — We  have  seen  this  arm 
fired  at  a  target  at  the  several  distances  of  300,  500,  600,  and 
700jards,  being  respectively  900,  1,500,  1,800  and  2,100  feet. 

The  target  was  a  pine  board  30  inches  square,  and  by  the 
inventor  was  hit  on  the  average,  twice  out  of  three  shots. 

By  experiments  and  calculations  lately  made  in  France,  it 
was  found  that  a  man,  at  the  distance  of  1,638  feet,  appears  to 
the  naked  eye  only  one-fifth  his  real  size,  and  therefore,  by  this 
estimate,  a  target  of  30  inches  in  diameter,  at  the  distance  of 
2,100  feet,  appears  less  than  six  inches  square,  a  small  object 
truly  in  practice,  and  requiring  an  accuracy  of  aim  so  minute, 
that  the  tenth  of  an  inch  in  the  direction  of  the  sight,  would 
carry  the  ball  far  aside  of  such  a  mark,  and  yet  it  was  pierced 
twice  out  of  three  shots. 

388.  Adjusted  Sight. — This  rifle   has   an   adjusting   sight, 
which  is  elevated,  or  depressed  and  fixed,  according  to  the  dis- 
tance of  the  mark.     All  the  shots  were  made,  with  the  gun  in 
the  hands,  or  without  a  rest,  and  also,  with  the  striking  pecu- 
liarity of  being  placed  on  the  left  shoulder. 

at  the  Distance  of  a  Mile. — Although  in  the  above 


398  SHARP'S  RIFLE. 

trial,  the  distance  was  only  700  yards,  the  inventor  has  proved 
by  experiment,  that  this  rifle  throws  its  ball  with  a  force  equal 
to  the  destruction  of  human  life  to  the  distance  of  a  mile. 

In  battle,  therefore,  the  approaching  enemy  can  be  effectually 
assailed  with  this  arm,  at  that  distance,  the  aim,  of  course,  being 
more  and  more  sure,  as  the  distance  diminishes. 

Number  of  Balls  Thrown. — The  rapidity  with  which  this  arm 
may  be  loaded  and  fired  is  such,  that  if  one  ball  be  sent  along 
the  surface  of  water,  another  may  be  made  to  follow  before  the 
first  ceases  its  motion. 

The  inventor  loads  and  fires  it  ten  times  in  a  minute ;  but 
estimating  that  in  battle  the  number  of  balls  fired  by  each  sol- 
dier would  be  only  six  in  a  minute,  then  1,000  men  would  dis- 
charge 6,000  in  a  minute,  or  360,000  in  an  hour. 

389.  Invention  of  Gunpowder. — In  Europe,  the  invention  of 
gunpowder  is  attributed  to  Roger  Bacon,  who  died  in  1292 ; 
but  it  seems  to  have  been  known  to  the  Chinese  long  before 
that  period. 

The  first  account  of  its  use  in  European  war,  was  at  the  bat- 
tle of  Cressy,  in  1346,  and  from  that  time  it  superseded,  chiefly, 
all  other  means  of  destruction  on  the  battle-field. 

Effects  of  this  Invention. — There  is  no  doubt  but  this  inven- 
tion has  proved  a  humane — a  merciful  discovery  in  the  art  of 
war. 

Before  its  use,  the  instruments  of  death  in  battle  were  the 
barbed  arrow,  the  halbert  and  spear,  various  kinds  of  swords, 
and  the  war-club. 

The  combatants  fought  hand  to  hand,  each  one  trying  to  in- 
flict the  most  cruel  tortures  on  the  other;  and, indeed,  the  in- 
struments employed,  were  much  better  calculated  for  this  pur- 
pose, than  for  the  infliction  of  sudden  and  quiet  death. 

On  the  contrary,  gun-shot  wounds,  when  not  instantly  fatal, 
afford  a  prospect  of  recovery,  while  those  made  by  the  barbed 
arrow  and  spear,  more  commonly  portend  a  miserable  death, 
after  protracted  agony. 

Besides,  if  we  examine  the  accounts  of  ancient  battles,  we 
shall  find,  that  including  the  carnage  on  the  field,  and  the  num- 
ber who  died  of  their  wounds  afterward,  the  destruction  of 
human  life,  where  an  equal  number  were  engaged,  was  much 
greater  than  it  was,  after  the  invention  and  use  of  gunpowder. 

390.  Conclusion. — Although  there  is  no  doubt  but  the  use 
of  fire-arms,  in  warfare,  has  heretofore  diminished  the  horrors 
of  the  battle-field,  this  circumstance,  as  history  informs  us,  has 


SHARP'S  RIFLE.  399 

had  no  influence  on  our  species,  except  to  foster  an  increasing 
desire  to  render  the  instruments  of  death  more  and  more  per- 
fect, so  that  in  the  day  of  battle,  the  carnage  should  be  as  sud- 
den and  as  great  as  possible.  And  hence,  within  a  few  years, 
great  improvements  have  been  made  on  fire-arms  in  France, 
England,  Germany,  and  America,  all  tending  of  course,  to  the 
increase  of  their  destructive  effects. 

The  inventors  of  these  improvements  in  the  arts  of  human 
destruction,  are  by  no  means  considered  by  political,  or  even 
by  moral  and  religious  writers,  as  enemies  of  the  human  race, 
but  are  viewed,  at  least,  by  many  such,  as  the  pioneers  of  uni- 
versal peace,  if,  indeed,  fallen  man  should  ever  cease  to  learn 
and  practice  the  art  of  war. 

391.  Settlement  by  Arbitration  Improbable. — The  history  of 
man  affords  no  foundation* for  the  belief  that  national  quarrels 
will  be  settled  by  the  intervention,  or  arbitration  of  other  nations, 
and  hence,  there  can  be  little  doubt,  if  the  moral  and  political 
condition  of  the  world  remain  as  heretofore,  that  "  nation  will 
continue  to  lift  up  its  arm  against  nation,"  and  that  the  time 
when  "  man  shall  learn  war  no  more,"  is  not  at  hand,  unless 
indeed,  it  should  be  by  the  approach  of  the  millenium. 

Under  this  view  of  the  case,  the  only  prospect  of  universal 
and  permanent  peace,  is  in  such  a  degree  of  perfection  in  the 
art  of  war,  that  certain  death  awaits  at  least  five  out  of  six  of  all 
who  enter  as  combatants  on  the  field  of  battle ;  and  in  naval  war- 
fare, an  equal  proportion  of  ships  shall  as  certainly  be  buried 
in  the  ocean. 

Until  such  a  state  of  things  exist,  men  will  continue  to  en- 
gage each  other  in  mortal  strife ;  and  it  is  on  this  account 
that  moralists  of  the  present  day,  look  with  favor  on  the  im- 
provements in  fire-arms,  knowing  that  the  paramount  design  of 
all  such  inventions  is  to  render  escape  more  difficult,  and  death 
more  sudden  and  certain  on  the  battle  ground. 

Nor  is  it  probable  that  the  time  is  far  distant,  when  at  least 
ten  will  foil  on  the  field,  where  with  an  equal  number  of  com- 
batants, only  one  fell  30  years  ago  ;  the  result  being  solely  from 
the  more  deadly  power  of  the  fire-arms  employed. 

The  author  having  served  as  surgeon  on  the  frontier,  in  the 
U.  S.  Army,  in  the  war  of  1812-15,  is  able  to  appreciate,  in  a 
measure,  the  difference  between  the  destructive  power  of  the 
fire-arms  then  furnished  by  the  government,  and  those  now  to 
be  introduced  into  the  Army  of  the  United  States. 

392.  Contrast  between  Old  and  New  Arms. — -To  those  who 


400 

have  examined  this  subject,  and  are  acquainted  with  the  arms 
employed  formerly,  and  those  now  going  into  general  use,  it 
will  not  be  considered  an  over  estimate  to  suppose,  that  100 
men  armed  with  Sharp's  rifle,  and  Colt's  revolvers,  would  com- 
mit greater  carnage,  on  the  battle  ground,  than  1,000  men 
could  do,  with  the  flint  lock,  government  muskets,  in  former 
use. 

393.  Who  indeed  will  enlist  into  a  military  service  when  he 
knows  that  his  enemy  will  oppose  him  with  messengers  of  death, 
at  the  rate  of  600  per  minute,  or  36,000  per  hour,  for  every 
100  men,  and  this  at  the  distance  of  a  mile,  or  less;  and  with 
the  same  number  of  such  messengers,  in  half  that  time,  when 
within  any  distance  between  20  yards,  and  the  reach  of  the 
bayonet ;  which  will  be  the  case  when  armies  are  supplied  with 
Sharp's  rifles,  and  Colt's  revolvers. 

From  such  sources  only,  according  to  the  present  aspect  of 
the  nations  of  the  earth,  can  we  look  for  permanent  peace. 

COLT'S   REPEATING    PISTOL. 

394.  This  celebrated  fire-arm  has  been  brought  to  its  present 
degree  of  perfection,  only  after  years  of  experience,  trial,  and 
invention,  by  the  original  patentee  Col.  Samuel  Colt,  of  Hart- 
ford, Conn. 

An  account  of  this  weapon  is  introduced  here,  as  an  inven- 
tion with  which  all  the  civilized  nations  of  the  earth,  are  now, 
or  are  soon,  to  become  acquainted. 

As  Americans,  therefore,  we  are  bound  to  know  something, 
at  least,  of  the  history  and  mechanism  of  so  important  an 
invention. 

The  examination  and  trial  of  Colt's  revolvers  at  the  World's 
Fair,  and  the  award  passed  upon  them  there  by  the  best  judges, 
and  the  most  experienced  military  men  of  the  age,  are  ample, 
and  sufficient  proofs  that  this,  for  the  purpose  for  which  it  is 
designed,  is  the  most  efficient  and  perfect  fire-arm  ever  invented. 

The  immense  demand  for  the  article  in  foreign  countries,  as 
well  as  in  our  own  country,  evinces,  also,  that  no  substitute 
exists  for  this  weapon. 

About  400  artificers,  we  understand,  are  employed  in  their 
manufacture,  which  number,  it  is  stated,  is  to  be  increased  to 
1000,  in  order  to  supply  the  demand. 

The  United  States  government  have  adopted  Colt's  repeat- 
ing pistol,  as  the  best  weapon  known,  for  mounted  men,  both 
for  offensive  and  defensive  use.  And  in  the  Mexican  war,  no 


COLT'S  PISTOL.  401 

officer  who  could  obtain  a  revolver,  ever  went  a  day  without 
one,  and  those  who  could  not,  often  considered  their  lives  in 
peril,  in  consequence. 

395.  In  1851,  the  President  of  the  United  States  in  a  mes- 
sage to  the  Senate,  states,  that 

"  Such  is  the  favorable  opinion  entertained  of  the  value  of  this 
arm,  particularly  for  mounted  corps,  that  the  secretary  of  war 
has  contracted  with  Mr.  Colt,  for  four  thousand  of  his  pistols," 
without  waiting  a  special  appropriation  of  Congress.  This 
contract,  of  course,  was  confirmed  by  the  Senate. 

Such  is  considered  the  importance  of  this  arm  as  a  weapon 
of  defence,  that  the  military  committee  of  the  House  of  Rep- 
resentatives, recommend  that  it  should  be  furnished  to  emi- 
grants, as  the  following  shows. 

"  We,  the  undersigned,  members  of  the  military  committee 
of  the  House  of  Representatives,  understanding  that  an  appli- 
cation is  pending  before  your  committee,  favorably  commended 
by  the  ordnance  department,  for  the  purchase  of  a  suitable 
number  of  Colt's  pistols,  and  authorizing  the  department  to 
furnish  the  same,  to  emigrants  at  government  prices,  and  to  de- 
liver the  same  to  the  States,  under  the  act  of  1808,  for  arming 
the  militia,  recommend  the  same  to  your  favorable  considera- 
tion, and  believe  that  such  a  clause  in  the  army  bill  would  be 
desirable  and  proper." 

Signed  by  the  committee,  nine  in  number,  January,  1851. 

396.  Description  of  Colt's  Pistol. — Why  these  arms  are 
called  revolvers,  and  by  what  means  they  are  made  the  most 
efficient  of  fire-arms,  for  certain  purposes,  will  be  understood 
by  Fig.  3 02,  and  the  following  explanation. 

The    letters    by 

which  the  principal  FIG-  302- 

parts  of  the  pistol 
are  denoted,  are  the 
following,  as  seen  on 
the  cut.  The  barrel 
B,  is  from  three,  to 
eight  inches  in  length 
according  to  the  size  cwfa  Pistol. 

and   design   of  the 

pistol.  The  cylinder  C,  is  the  part  which  revolves,  and  from 
which  the  arm  takes  its  distinctive  name.  The  mechanism  by 
which  the  rotary  motion  is  performed,  can  not  be  shown  by  a 
single  figure.  The  cylinder  is  pierced  with  six  apertures,  called 


402  MC  CORMICK'S  REAPER. 

chambers,  each  of  which,  when  ready  for  action,  contains  a 
charge  of  powder  and  a  ball.  Caps  are  then  put  on  the  tubes, 
corresponding  to  each  charge,  and  now  the  arm  is  ready  for  the 
discharge  of  six  balls,  as  rapidly  as  the  hammer  can  be  drawn 
to  the  back  notch  and  the  trigger  pulled. 

The  hammer  H,  being  drawn  back  to  where  it  now  stands,  is 
made  to  strike,  with  its  face,"  the  cap  on  the  tube,  by  which  it 
is  exploded,  and  the  pistol  discharged.  Then  on  drawing  the 
hammer  back  for  another  discharge,  the  mechanism  makes  the 
cylinder  revolve  one  notch,  by  which  the  next  cap  is  brought 
under  the  hammer,  and  by  pulling  the  trigger  is  discharged, 
and  so  of  all  the  other  charges.  The  trigger  requires  no  expla- 
nation, being  in  all  tire-arms  the  same.  The  ramrod  R,  is  con- 
nected with  the  lever  L,  by  the  united  action  of  which,  the  ball 
is  pushed  down  the  chamber  to  the  powder. 

397.  Having  explained  the  references,  we  will  give  tho 
inventors  own  directions  for  loading,  &c. 

"  Draw  back  the  hammer  to  the  half  notch,  which  allows  the 
cylinders  to  be  rotated ;  a  charge  of  powder  is  then  placed  in 
each  chamber,  and  the  balls,  without  wadding,  or  patch,  are 
put,  one  at  a  time,  upon  the  mouths  of  the  chambers,  turned 
under  the  rammer,  and  forced  down  with  the  lever  below  the 
mouth  of  the  chamber.  This  is  repeated  until  all  the  cham- 
bers are  loaded.  Percussion  caps  are  then  placed  on  the  tubes, 
when,  by  drawing  back  the  hammer  to  the  full  catch,  the  arm 
is  in  condition  for  a  discharge  by  pulling  the  trigger ;  a  repeti- 
tion of  the  same  motion  produces  like  results." 

When  this  arm  is  prepared,  therefore,  all  that  is  required  in 
defence,  or  in  action,  is  to  draw  back  the  hammer,  and  pull  the 
trigger,  until  the  six  balls  are  discharged,  which  is  done  in  less 
than  half  a  minute. 


MC  CORMICK'S  REAPER. 

398.  The  principal,  or  cutting  apparatus  of  this  famous  ma- 
chine, is  shown  by  Fig.  303.  The  entire  machinery,  consisting 
not  only  of  the  four  wheels  on  which  the  whole  rests,  but  also 
of  bands,  cranks,  cog-wheels,  driver's  seat,  and  platform  for  the 
grain — the  whole  being  connected  and  supported  by  braces  in 
all  directions ;  it  is  obvious,  is  too  complicated  an  engine  for 
the  purposes  of  a  school  book. 

Nor  are  these  parts  necessary  to  show  the  mystery,  in  which 
the  public  are  chiefly  interested,  viz.,  how  it  is  possible  that  a 


MO  CORMICK  S   REAPER. 
FIG.  803. 


403 


McCormick's  Reaper. 

machine  drawn  by  horses,  can  do  what  only  the  hands  of  man 
have  heretofore  performed  with  the  sickle  and  cradle. 

The  above  drawing  is  designed  merely  to  illustrate  and  ex- 
plain this  wonder. 

The  angular  pointed  projections,  marked  by  numbers  1,  2,  3, 
4,  and  5,  are  called  the  fingers.  They  are  firmly  driven  into  a 
beam  of  wood,  at  the  distance  of  4-£  inches  from  the  center  of 
one  to  that  of  the  other,  and  their  length  is  about  the  same 
number  of  inches.  They  are  of  cast  iron,  without  cutting  edges. 

At  the  base  of  the  fingers,  and  between  their  angles,  are  seen 
the  sickles,  angular  in  form,  and  composed  of  sections  of  steel 
plate  riveted  to  an  iron  strap,  about  an  inch  wide,  which  strap 
is  movable  to  the  right  and  left  on  the  beam  of  wood  into  which 
the  fingers  are  driven. 

399.  The  sickles  have  thin  cutting  edges,  which  are  finely 
serrated,  similar  to  a  common  sickle,  the  teeth  standing  right 
and  left  from  the  center  or  angle  of  each. 

While  the  fingers  are  fixed  to  the  beam,  the  sickles  have  a 
reciprocating  motion  of  about  4  inches,  alternately  to  right  and 
left  by  means  of  a  crank,  turned  by  the  force  of  the  wheels,  by 
which  the  whole  machine  is  moved. 

This  is  the  effective  or  cutting  portion  of  McCormick's  reaper. 
All  the  other  parts  are  adjuvants  to  this,  being  the  means  by 
which  this  is  moved  and  actuated. 

The  divider  A,  is  a  piece  of  iron  which  extends  forward  of 
the  fingers,  and  is  designed  to  separate  the  grain  to  be  cut  from 
that  which  is  to  be  left  standing.  This,  as  its  shape  indicates, 


404 

bends  the  grain  to  be  cut  inwards,  leaving  that  which  remains 
in  a  well-defined  and  perfect  line,  until  the  return  of  the  reaper. 

The  strips  of  wood,  B,  fastened  to  the  beam  on  which  the 
sickles  work,  show  where  the  force,  by  means  of  horses,  is  ap- 
plied, and  by  which  the  whole  is  drawn  on  four  wheels  of  mod- 
erate size. 

400.  Action  of  the  Reaper. — It  will  be  observed  that  the 
effective  or  cutting  portion  of  the  machine,  extends  to  the  right 
of  the  place  where  the  moving  force  is  applied,  and  hence  that 
the  horses  work  on  the  side  of  the  standing  grain. 

The  grain,  therefore,  is  cut  by  beginning  on  the  outside  and 
going  around  the  field,  the  horses  passing  that  which  has  been 
cut,  while  the  sickles  extend  about  six  feet  into  that  which  is 
standing. 

The  cutting  is  performed  by  the  alternate,  or  reciprocating 
motions  of  the  sickles  against  the  grain,  which  is  kept  from  re- 
ceding by  the  oblique,  angular  form  of  the  fingers,  as  shown 
by  the  figure,  after  the  inspection  of  which,  no  further  descrip- 
tion will  be  required  to  show  how  the  operation  is  performed. 

As  the  grain  is  cut,  it  falls  upon  a  platform,  where  a  man 
stands,  with  a  rake,  to  gather  and  remove  it  to  the  outside  of 
the  machine,  and  where  it  is  bound  by  men  who  follow  for  this 
purpose.  Thus  the  way  is  cleared  for  the  return  of  the  horses 
and  reaper. 

It  is  stated  that  the  fields  of  wheat  thus  cut,  present  a  very 
smooth  and,  to  the  eyes  of  the  farmer  and  others,  beautiful  ap- 
pearance— the  stubble  presenting  a  level  and  even  surface 
throughout. 

The  inventor  of  this  reaper  not  only  received  the  highest  pre- 
mium at  the  World's  Fair,  in  England,  but  also  the  gold  medal, 
in  the  States  of  Ohio  and  Illinois,  for  the  most  complete,  and 
best  working  machine  of  this  kind  presented. 

It  is  stated  that  such  is  the  demand  for  this  reaper,  that  sev- 
eral thousands  will  probably  be  sold  in  the  course  of  the  present 
year.  The  price  is  about  $120. 

The  inventor  also  constructs  mowing  machines,  on  the  same 
principle  as  the  reaper. 


INDEX. 


A. 

Ball,  cannon,  velocity 
of,  58,60 

Colt's  revolver,  400 

Comets,  335 

ACOUSTICS,  198 
Accidents  189 

path  of,  58 
Barkers'  mill,  131 
Battery,  galvanic,  358 
tobacco  pipe,  360 
Biot,  on  sound,  200 
Bodies,  properties  of,  .  .       7 
fall  of  li"ht,                35 

Concave  mirror,  effects 
of,   239 

to  the  rails,  196 

Counter  currents,  214 
Convex  lens,  243 
Condenser,  148 
Cornea  too  convex,  ....  251 
Cornea,  too  flat,  252 

Action  and  reaction,  .  .     40 
^Eolian  harp         205 

Air,  elasticity  of,  139 

expansion  of,  140 
in  every  crevice,  .  .  138 
compression  of,  ...  139 
weight  of,  140 

n      S>    „ 

Constellations,  283 

Body,  definition  of,  ....       7 

Crank,  nature  of,  181 

Artificial  magnet  353 
Ascent  of  bodies,  32 
Air-ffun,  149 

Boat',  and  bellows,  41 
Battery,  galvanic,  358 
Grove's                       360 

Cup  and  shilling        ...  2ii2 

Cylinder  steam         .  .  .  178 

D. 

pump,  1** 

double-acting,  143 
experiments  with,  .  14 
Atmospheric  pump,  .  .  .   160 
Atmosphere,  pressure  of,  146 
phenomena  of,  140 
conveys  sound,  198 
Atmospheric  electricity,  349 
Anemometer,  215 

construction  of,  ...  150 
use  of,  at  sea,  159 
wheel                          155 

Day  and  night,'  310 

weather  glass,  157 

Decomposition,  10 
Definition,  172 

measures  heights,  .  156 

Differential  thermometer.  173 
Density,    20 

diminution  of,  ....  157 
of  the  planets  285 
Dick  Thomas               ..  264 

Ascent  of  bodies,  32 

Brittleness,'  21 

Astraea,  **a 
Attraction  in  general,..     12 
of  built  16 

Brick,  reading  through  a,  241 
Burning  glass  240 

Divisibility  of  matter,  .  .       9 
Dioptrics,  221 
Diving  bell                         199 

capillary  13,  J  7 
of  cohesion,  13 
chemical                      18 

C. 

Caloric  engine,  379 
Camera  obscura,  .  .  266,  380 
Cask,  bursting  of,  107 

Ductility,  .'  22 
Double  refraction,  224 
Dipping  needle,.  .  .  352,  355 

E. 

Tort  I,                                                       200 

of  gravitation,  15 
electrical,  20 

magnetic,  ....     19,  352 
proportionate  to  mat- 
ter,       30 

Capstan,  82 

Card  machine,  100 

axis  of,  290 

Angle  of  vision,...  245,254 

Cannon  ball,  velocity  of,    60 
fall  of                          58 

distance  from  sun,.  290 
diurnal  motion  of,.  290 
revolution  of,  290 
falling  to  Sun  29 
form  of,  314 

definition  of,  276 
physical,  276 

revolution  of,  59 

practical,  276 

Ceres         '                        278 

Archimedes'  screw,  128 
Asteroids  277 

Centrifugal  force,  .     46,  316 
Centripetal  force,  46 
Center  of  gravity,  48 

velocity  of,  311 

Ecliptic                            282 

Atwoods'  machine,  28 
Axis  of  a  planet,  280 
of  the  earth,  290 

lunar,    325 

Chronomete'r                     332 

solar,  328 

Echo  325 

B. 
Balance,  117 

Chromatics,  269 
Clock,  common,  66 
Clio                                  278 

Egeria,  278 
Electricity            339 

theories  of,  341 

Circus  rider,  45 

Electrical  machine,  ....  342 

Ball,  movement  of,.  .  11,  50 
revolution  of,  59 

Coal,  power  of,  191 
Colors  of  objects,  272 

attraction,  21 
battery,  348,356 
helix.  ...              .  .  368 

406 


INDEX. 


Elasticity,  20 

Electrical  telegraph,...  386 

bodies, :.....  339 

Electro-magnetism.  . . .  357 

laws  of; 357 

motion  of, 361 

Electroscope, 361 

Electrotype, 370 

Electrometer, 346 

Electro-gilding, 376 

plating, 377 

Engine,  steam, 174 

Engine,  atmospheric,  . .  175 

caloric, 379 

fire, 165 

Equal  forces, 32 

Equation  of  time, 318 

Equilibrium, 55 

Erect  image, 250 

Extension, 8 

Eye,  human 248 

of  an  ox, 250 

F. 

Falling  bodies,...         .  26 

light 35 

direction  of, 25 

velocity  of, 26 

Figure  of  bodies, 8 

of  the  earth 315 

Fire  engine, 165 

Five  mechanical   pow- 
ers   101 

Flora, 278 

Fluids,  what 103 

discharge  of, 124 

Focus,  principal, 237 

Form,  change  of, 21 

Fly-wheel,  184 

Focal  distance,  ...  232,  243 

'Force,  what, 71 

equal, 31 

not  created, 86 

of  gravity, 12 

Fountain  of  Hiero, 166 

expansion, 147 

Friction  of  machinery,.  86 

of  fluids, 126 

Fulcrum,   72 

G. 

Galvanism, 357 

Gallery,  whispering,  ...  202 

Galvanic  battery, 358 

Grove's, 360 

Globular  form, 14 

Gold  leaf, 9 

G  overnor,  steam, 185 

Gravity  terrestrial,  ....  24 

force  of, 24 

Gravitation, 15 

Gravity,  center  of, 48 

in  man 52 

how  taken, 48 

specific,  .- 116 

table  of, 118 

Gregory's  telescope,  . . .  263 

Gun,  air, 149 


59 

H. 

Harmonicon,  

211 

Hay,  load  of,  

52 

Heat,  absorption  of,  ... 

167 

distribution  of,  .  .  .  . 

167 

by  concave  mirror, 

240 

radiation  of,  

167 

reflection  of,  

167 

transmission  of,  ... 

168 

20 

Harp,  ajoliuu  

205 

Hebe  

278 

Herschel,  planet,  

296 

his  telescope,  

2G3 

Hiero's  fountain,  

166 

Helix,  electrical,  

368 

High  pressure  engine,.. 
Hoblyn,  Prof.,  

188 
174 

Horizon,  

189 
308 

Horology,  
Hydraulics,  

65 
123 

Hydrostatic  bellows,  .  .  . 

110 

press,  

110 

Human  face  magnified, 

238 

Hydrometer,  

119 

Hydrostatics,  

136 
102 

Hydrostatic  paradox,  .  . 

108 

Hygrometer,  

173 

itygeia,  

278 

I. 

Impenetrability,  
Inertia,  center  of,  

7 
54 

Imp,  bottle,  
Inclined  plane,  

150 
93 

motion  on,  

93 

256 

Indestructibility,  
Instruments,  musical,.. 

9 

204 

Irene  

278 

Iria  

278 

3. 

Juno,  

278 

Jupiter,  

292 

293 

distance  of,  

292 

K. 

80 

L. 

268 

Latitude,  what,  

329 

how  found,  

330 

Lenses,  what,  

243 

forms  of,  

243 

refraction  by,  
Leyden  jar,  

242 
348 

51 

Lens,  concave,  

245 

convex,  

243 

double-convex,.... 

244 

Level,  water 115 

spirit, 116 

Lever,  what 72 

simple,  72 

compound, 78 

compared, 78 

knee, 80 

Lightning  rods, 350 

Light,  convergent  rays,  237 

diverging  rays  of,. .  244 

refraction  of, 221 

reflection  of, 225 

decomposition  of,  .  269 

motion  of, 220 

re-composition  of, .  270 

velocity  of, 220 

Locomotive, 192 

boiler, 197 

Locomotive,  described,  193 

Longitude,  what, 330 

how  found, 332 

Lunar,  eclipses, 324 

M. 

Machine,  what, 70 

for  raising  water, . .  128 

Mars 291 

Malleability, 22 

Magic  lantern 268 

Machinery,  use  of,  ....     70 

Magnetism,  351 

Magnitudes,  judged  of,.  254 
Magnets,  artificial,  ....  353 

temporary, 355 

Magnetic  needle, 355 

rotation, 354 

dip  of, 287 

Magdeburg  hemispheres,  146 

Mechanics, 70 

Mercury, 383 

Metronome, 69 

McCormick's  reaper, . . .  402 

Metis 278 

Microscope,  simple, 256 

compound 257 

solar, 258 

Momentum, 39 

Mountain,  rupture  of,. .  112 
Mechanical  powers,  ...     72 

Mirrors,  what, 226 

concave, 235 

focus  of, 237 

convex, 229 

metallic, 240 

plane, 226 

plane  inclined, 235 

Morse's  telegraph, 383 

Moon, 291 

fall  to  earth, 29 

phases  of, 321 

surface  of, 323 

eclipses  of, 384 

Motion,  what, 36 

absolute, 37 

axis  of,  43 

center  of, 46 

compound 43 

circular, 45 


INDEX. 


407 


Motion,  crank,  182 

Perkins'  experiments,  .  .  103 

Sound,  propagation  of,    200 
reflection  of              201 

curvilinear  55 
diagonal,  44,  64 
parallel  179 
reflected,  41 

cylinder,  393 

Prismatic  spectrum,  .  .  .  269 
Properties  of  bodies,  ...       7 
Projectiles                      .     60 

reverberation  of,  .  .  200 
velocity  of,  200 
Solar  spectrum,  269 
system,  276 

relative,   37 
resultant,  63 
Motion,  retarded,  38 

Pump,  air,  142 
atmospheric,  160 
chain  132 

Spectacles,  247,  245 
Summer  and  winter,.  .     313 
Specific  gravity,  116 
Spring,  intermitting,..     122 
System  of  pulleys,  89 
Steel-yard,  75 

uniform,  3 

forcin"  '                    163 

vertical,   33 

lifting?  161 

String,  vibration  of,  .  .     204 

Musical  strings,  204 
instruments,  204 
Monochord  206 

stomach,  16 

Steam  cylinder,...  178,186 

water,'  159 

N. 
Neptune,   297 

engine,  174 
modern,   187 

low  pressure,  188 
high  pressure,  188 

White's,'-  92 

New  plant,  278 
Needle  dipping                 355 

R. 

Rails,  adhesion  to,  ....  196 
Rain  216 

Watt's,  176 
Newcomen's,  176 

0. 

Objects  seen  erect,  253 

btwna      pump,            ..  286 

distance  of,  286 
eclipses  of,  328 

Optics,  218 
Optics,  definition  of,  ...  218 
Optical  instruments,...  256 
definitions,  219 

guage,  217 
Ravs,  convergent,  229 
'divergent,  230 
Rainbow,  272 

revolution  of,  286 

Siphon  121 

Orbit,  what  280 
elliptical         "            281 

Rariety,  20 
Reaper,  McCormick's,  .  402 
Re«t                       ..       .37 

T. 

Table  of  velocities,  38 
Temporary  magnets,  ...  259 
TeleJcope,..  258 

Organ              207 

construction  of,  .  .     208 
invention  of,  210 
ant  quity  of,  210 
larae,    ..  210 

Revolver,  Colt's  400 
Revolving  bell  engine,  .  364 

whiel         36-2 

pipV...  208 
P. 

Revolution  of  wheels,  .  .     47 
oftheearth,  309 

principle  of,  260 

refracting                   260 

of  planets,  59 

381 

Pallas   278 
Paradox,  hydrostatic,  .  .  109 

Reflection  bv  mirrors,.  .  225 
of  sound  201 

Telescope,  reflecting,  .  .  262 
Rosse's,  264 

Telegraph                   .  .  .  386 

.  annual,  338 
diurnal,  338 

Refraction,  what,  221 
oflight,  223 
laws  of,  222 

House's  383 

Pipes,  organ  208,  209 
Piaster,  casts  of,  374 
Plane,  inclined,  93 
Planetoids,  279 

double,  224 
by  glass,  223 
by  water      222 

ofwood  22 
of  metals,  23 
Thermo-electricity,  ....  369 
solar  .  317 

Retina,  248,250 

distances  of,  277 
density  of                   285 

Recapitulation,  23 
Rifle.  Sharp's,  395 
Rotation  of  a  wheel,  .  .     4"t 
River,  currents  in,  12"* 
Rosse's  telescope,  264 

3. 
Saturn,  294 

alcoholic,  169 
comparison  of,  ....  170 
Rutherford's,  172 
Leslie's,  172 

motion  of,  277 

situation  of            .  279 

table  of                      278 

Tides,                                 327 

u    ,  .  . 

Trade  wind,    212 

Penumbra,  326 

U. 

Umbra,                             326 

Phenomena,  atmospher- 

Scales       75 
Seasons  310 

ic,   212 
379 

Philosophy  defined,....       7 
Piledriver  40 

heat  and  cold  of,  .  .  312 
Screw          96 

V. 

Variation  magnetic,...  356 

Archimedes',  12t 
perpetual,  9J 

Power,  what,  71 

power  of,  97 

Shepherds  of  Landes,  .  .     5J 

table  of,  356 

Velocity  of  fall  ing  bodies,  2t 
accelerated,  38 

varying,     84    Dmae  »  U8llcrjr  "* 

408 


INDEX, 


VeJbcity  of  a  ball,, 
retarded       .   . 

....     60 
....     38 

Visual  angle,  

254 
206 
196 
347 

84 
103 
103 

106 
112 
128 
114 
223 
128 
127 
159 
114 
117 
136 

Water  wheels,  .... 

.     133 

Vibration  of  cords,  .... 
of  solids,  

Weight,  what,  

24,47 
..     85 
..     47 

of  the  earth 

.  311 

Wheels,  system  of,  . 
Wheel,  revolving,  .  . 

of  light 

220 

Vial,  Leyden,  

37 

W. 
Watch-work  

flv 

184 

of  certain  bodi 
table  of,     ,    ., 
of  wind 

es...     38 
38 
216 

134 

breast,    ... 

135 
..  130 

of  electricity, 
V  enus 

..  337 
288 

Water,  what,  
elasticity  of,  
equal  pressure    of, 
104, 

Wheel  and  axle 

..     85 
..     47 

289 

Wed^e 

96 

evening  star,.. 

....  290 

Whispering  gallery,  . 
Wind  instruments,  .  . 
Wind,  what,  

..  202 
..  207 
..  2J2 
213 

morning  star,  . 
Vibrating  wire,  .  .  . 

....290 
....  363 
245,  253 
242 
....  251 

friction  of,  

level,  

angle  of,  
perfect,  

refraction  by,  
raising  of,  

velocity  of, 
Windlass,  

215 
..     83 

indistinct, 
Vertical  motion,  .  . 

V«rtn 

251 
256 
33 

278 

pumps,  

table  of  pressure,  .  . 
weighing  in,  

Z. 
Zodiac,  

..  283 

7b           ram,  

ERRATA,  Astronomy,  p.  114,  insert  the  Asteroids  as  in  p.  120.  In  p. 
122,  omit  the  last  clause,  and  look  to  Saturn  for  the  correction.  The  rela 
tive  size  of  Neptune  can  not  be  shown  on  the  page. 


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